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Fair Value Measurements - Financial Assets Measured on Recurring Basis (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments, available for sale, at fair value	$ 14.9	$ 0.0
Catastrophe Bonds, Fair Value Disclosure	1.6	5.1
Derivatives at fair value	31.7	56.2
Fair market value	4,122.6	3,788.6
Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	2.1	6.3
Commercial mortgage loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	274.9	312.1
Middle market loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	84.8	106.9
Equity securities — Trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)		6.6
Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	18.0	25.6
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	117.1	134.6
Privately-held Investments trading, at fair value	475.0
Total	6,410.7	6,100.9
Recurring | Derivatives at fair value | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value	31.7	56.2
Recurring | Liabilities under derivative contracts | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value	(9.3)	(3.2)
Recurring | Liabilities under derivative contracts | Loss Portfolio Transfer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value	(16.5)	(31.7)
Recurring | Total fixed income securities — Available for sale | Agency commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	5.8
Recurring | Fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	4,122.6	3,788.6
Recurring | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	93.6	52.0
Recurring | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,485.7	1,475.5
Catastrophe Bonds, Fair Value Disclosure	1.6	2.9
Recurring | Short-term investments trading at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2.1
Recurring | Real estate fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	209.3	221.3
Recurring | Privately-held investments, Available for Sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments, available for sale, at fair value	14.9
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value	0.0
Total	1,748.4	1,428.7
Recurring | Level 1 | Derivatives at fair value | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value		0.0
Recurring | Level 1 | Liabilities under derivative contracts | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value	0.0	0.0
Recurring | Level 1 | Liabilities under derivative contracts | Loss Portfolio Transfer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value	0.0	0.0
Recurring | Level 1 | Total fixed income securities — Available for sale | Agency commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0
Recurring | Level 1 | Fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,403.0	1,098.3
Recurring | Level 1 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	86.7	51.9
Recurring | Level 1 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	258.5	272.2
Catastrophe Bonds, Fair Value Disclosure	0.0	0.0
Recurring | Level 1 | Short-term investments trading at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.2
Recurring | Level 1 | Real estate fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Recurring | Level 1 | Privately-held investments, Available for Sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments, available for sale, at fair value	0.0
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value	0.0
Total	3,979.6	3,949.6
Recurring | Level 2 | Derivatives at fair value | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value	31.7	56.2
Recurring | Level 2 | Liabilities under derivative contracts | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value	(9.3)	(3.2)
Recurring | Level 2 | Liabilities under derivative contracts | Loss Portfolio Transfer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value		0.0
Recurring | Level 2 | Total fixed income securities — Available for sale | Agency commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	5.8
Recurring | Level 2 | Fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2,719.6	2,690.3
Recurring | Level 2 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	6.9	0.1
Recurring | Level 2 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,227.2	1,203.3
Catastrophe Bonds, Fair Value Disclosure	1.6	2.9
Recurring | Level 2 | Short-term investments trading at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1.9
Recurring | Level 2 | Real estate fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Recurring | Level 2 | Privately-held investments, Available for Sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments, available for sale, at fair value	0.0
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value	475.0
Total	473.4	501.3
Change in unrealized investment gains (losses)	(15.7)	(4.6)
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	489.9	533.0
Recurring | Level 3 | Liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value	(16.5)	(31.7)	$ 0.0
Liability purchases	0.0	(17.2)
Liability transfers	0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3	0.0	0.0
Liability sales	0.0	0.0
Liability, increase (decrease) in fv included in net income	15.2	(14.5)
Change in unrealized gains or losses	15.2	0.0
Recurring | Level 3 | Loss Portfolio Transfer | Liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value	(16.5)	(31.7)	0.0
Liability purchases	0.0	(17.2)
Liability transfers	0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3	0.0	0.0
Liability sales	0.0	0.0
Liability, increase (decrease) in fv included in net income	15.2	(14.5)
Change in unrealized gains or losses	15.2	0.0
Recurring | Level 3 | Derivatives at fair value | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value		0.0
Recurring | Level 3 | Liabilities under derivative contracts | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value	0.0	0.0
Recurring | Level 3 | Liabilities under derivative contracts | Loss Portfolio Transfer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value	(16.5)	(31.7)
Recurring | Level 3 | Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and issuances	113.7	378.2
Transfers in/(out)	5.3	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(12.1)	0.0
Settlements and sales	(137.2)	(147.4)
Increase (decrease) in fair value included in net income	(12.8)	(4.9)
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)		533.0	307.1
Recurring | Level 3 | Total fixed income securities — Available for sale | Agency commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0
Recurring | Level 3 | Fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Recurring | Level 3 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Recurring | Level 3 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Catastrophe Bonds, Fair Value Disclosure	0.0	0.0
Recurring | Level 3 | Short-term investments trading at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0
Recurring | Level 3 | Real estate fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Recurring | Level 3 | Commercial mortgage loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in unrealized investment gains (losses)	(17.9)	(0.5)
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	274.9	312.1
Recurring | Level 3 | Commercial mortgage loans | Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and issuances	40.6	215.7
Transfers in/(out)	0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0.0	0.0
Settlements and sales	(61.5)	(113.1)
Increase (decrease) in fair value included in net income	(16.3)	(2.0)
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)		312.1	211.5
Recurring | Level 3 | Middle market loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in unrealized investment gains (losses)	0.5	(2.1)
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	84.8	106.9
Recurring | Level 3 | Middle market loans [Member] | Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and issuances	18.3	61.8
Transfers in/(out)	0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0.0	0.0
Settlements and sales	(41.9)	(19.3)
Increase (decrease) in fair value included in net income	1.5	(0.9)
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)		106.9	65.3
Recurring | Level 3 | Global corporate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in unrealized investment gains (losses)	(0.2)	0.0
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	14.4	15.0
Recurring | Level 3 | Global corporate securities | Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and issuances	0.0	15.1
Transfers in/(out)	0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0.0	0.0
Settlements and sales	(0.4)	0.0
Increase (decrease) in fair value included in net income	(0.2)	(0.1)
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)		15.0	0.0
Recurring | Level 3 | Equity securities — Trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in unrealized investment gains (losses)	0.0	0.0
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	0.0	6.6
Recurring | Level 3 | Equity securities — Trading | Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and issuances	0.0	5.5
Transfers in/(out)	0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(6.6)	0.0
Settlements and sales	0.0	(2.4)
Increase (decrease) in fair value included in net income	0.0	(0.1)
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)		6.6	3.6
Recurring | Level 3 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in unrealized investment gains (losses)	0.0	0.0
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	18.0	25.6
Recurring | Level 3 | Short-term investments | Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and issuances	18.2	25.6
Transfers in/(out)	0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0.0	0.0
Settlements and sales	(25.8)	0.0
Increase (decrease) in fair value included in net income	0.0	0.0
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)		25.6	0.0
Recurring | Level 3 | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in unrealized investment gains (losses)	1.7	(2.0)
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	82.9	66.8
Recurring | Level 3 | Asset-backed | Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and issuances	21.9	54.5
Transfers in/(out)	5.3	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(5.5)	0.0
Settlements and sales	(7.6)	(12.6)
Increase (decrease) in fair value included in net income	2.0	(1.8)
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)		66.8	$ 26.7
Recurring | Level 3 | Privately-held investments, Available for Sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments, available for sale, at fair value	14.9
U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	1,202.6	953.0
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	245.5	261.6
U.S. government | Recurring | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,202.6	953.0
U.S. government | Recurring | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	245.5	261.6
U.S. government | Recurring | Level 1 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,202.6	953.0
U.S. government | Recurring | Level 1 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	245.5	261.6
U.S. government | Recurring | Level 2 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
U.S. government | Recurring | Level 2 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
U.S. government | Recurring | Level 3 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
U.S. government | Recurring | Level 3 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	7.2	8.8
U.S. agency | Recurring | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	7.2	8.8
U.S. agency | Recurring | Level 1 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
U.S. agency | Recurring | Level 2 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	7.2	8.8
U.S. agency | Recurring | Level 3 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	128.1	149.5
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	3.1	3.6
Municipal | Recurring | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	128.1	149.5
Municipal | Recurring | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	3.1	3.6
Municipal | Recurring | Level 1 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Municipal | Recurring | Level 1 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Municipal | Recurring | Level 2 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	128.1	149.5
Municipal | Recurring | Level 2 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	3.1	3.6
Municipal | Recurring | Level 3 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Municipal | Recurring | Level 3 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	1,959.3	1,845.0
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	171.5	162.1
Corporate | Recurring | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,959.3	1,845.0
Corporate | Recurring | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	171.5	162.1
Corporate | Recurring | Level 1 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Corporate | Recurring | Level 1 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Corporate | Recurring | Level 2 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,959.3	1,845.0
Corporate | Recurring | Level 2 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	171.5	162.1
Corporate | Recurring | Level 3 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Corporate | Recurring | Level 3 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Non-U.S. government-backed corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	100.7	110.4
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	8.3	11.6
Non-U.S. government-backed corporate | Recurring | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	100.7	110.4
Non-U.S. government-backed corporate | Recurring | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	8.3	11.6
Non-U.S. government-backed corporate | Recurring | Level 1 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Non-U.S. government-backed corporate | Recurring | Level 1 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Non-U.S. government-backed corporate | Recurring | Level 2 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	100.7	110.4
Non-U.S. government-backed corporate | Recurring | Level 2 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	8.3	11.6
Non-U.S. government-backed corporate | Recurring | Level 3 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Non-U.S. government-backed corporate | Recurring | Level 3 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Non-U.S government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	273.8	213.6
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	34.8	30.4
Non-U.S government | Recurring | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	273.8	213.6
Non-U.S government | Recurring | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	34.8	30.4
Non-U.S government | Recurring | Level 1 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	200.4	145.3
Non-U.S government | Recurring | Level 1 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	13.0	10.6
Non-U.S government | Recurring | Level 2 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	73.4	68.3
Non-U.S government | Recurring | Level 2 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	21.8	19.8
Non-U.S government | Recurring | Level 3 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Non-U.S government | Recurring | Level 3 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	908.2	896.5
Asset-backed | Recurring | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	908.2	896.5
Asset-backed | Recurring | Level 1 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Asset-backed | Recurring | Level 2 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	908.2	896.5
Asset-backed | Recurring | Level 3 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Agency commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	5.8	5.6
Agency commercial mortgage-backed securities | Recurring | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		5.6
Agency commercial mortgage-backed securities | Recurring | Level 1 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0.0
Agency commercial mortgage-backed securities | Recurring | Level 2 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		5.6
Agency commercial mortgage-backed securities | Recurring | Level 3 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0.0
Residential Mortgage-Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	445.1
Residential Mortgage-Backed Securities | Recurring | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	445.1
Residential Mortgage-Backed Securities | Recurring | Level 1 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0
Residential Mortgage-Backed Securities | Recurring | Level 2 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	445.1
Residential Mortgage-Backed Securities | Recurring | Level 3 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0
Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	445.1	502.7
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	22.2	21.4
Agency mortgage-backed | Recurring | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		502.7
Agency mortgage-backed | Recurring | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	22.2	21.4
Agency mortgage-backed | Recurring | Level 1 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0.0
Agency mortgage-backed | Recurring | Level 1 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Agency mortgage-backed | Recurring | Level 2 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		502.7
Agency mortgage-backed | Recurring | Level 2 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	22.2	21.4
Agency mortgage-backed | Recurring | Level 3 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0.0
Agency mortgage-backed | Recurring | Level 3 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
High yield loans | Recurring | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	92.1	88.3
High yield loans | Recurring | Level 1 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
High yield loans | Recurring | Level 2 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	92.1	88.3
High yield loans | Recurring | Level 3 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Held for trading financial assets, at fair value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value		533.0
Held for trading financial assets, at fair value | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value		0.0
Held for trading financial assets, at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value		0.0
Held for trading financial assets, at fair value | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value		533.0
Short-term investments trading at fair value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		6.3
Short-term investments trading at fair value | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		6.3
Short-term investments trading at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0.0
Short-term investments trading at fair value | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0.0
Asset backed securities, privately-held afs | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	14.9
Change in unrealized investment gains (losses)	0.2
Asset backed securities, privately-held afs | Recurring | Level 3 | Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value		$ 0.0
Purchases and issuances	14.7
Transfers in/(out)	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0.0
Settlements and sales	0.0
Increase (decrease) in fair value included in net income	$ 0.2

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