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Fair Value Measurements - Financial Assets Measured on Recurring Basis (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments, available for sale, at fair value	$ 24.2	$ 14.9
Catastrophe Bonds, Fair Value Disclosure	1.0	1.6
Derivative assets	17.0	31.7
Fair Market Value	4,978.3	4,231.1
Fair Market Value	1,488.7	1,964.4
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	111.7	117.1
Privately-held Investments, at fair value	286.8
Total	6,701.7	6,410.7
Recurring | Derivatives at fair value | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	17.0	31.7
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	(45.9)	(9.3)
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	(3.6)	(16.5)
Recurring | Fixed income securities | Non-agency commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	4.4
Recurring | Fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	4,692.2	4,122.6
Recurring | Short-term investments — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	261.9	93.6
Recurring | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,199.9	1,485.7
Catastrophe Bonds, Fair Value Disclosure	1.0	1.6
Recurring | Short-term investments trading at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1.0
Recurring | Real estate fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	267.2	209.3
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	24.2
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments, at fair value	0.0
Total	2,207.8	1,748.4
Recurring | Level 1 | Derivatives at fair value | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0.0	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 | Fixed income securities | Non-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	1,675.7	1,403.0
Recurring | Level 1 | Short-term investments — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	260.2	86.7
Recurring | Level 1 | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	270.9	258.5
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	1.0
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, at fair value	0.0
Total	3,919.3	3,979.6
Recurring | Level 2 | Derivatives at fair value | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	17.0	31.7
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	(45.9)	(9.3)
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	0.0
Recurring | Level 2 | Fixed income securities | Non-agency commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	4.4
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	3,016.5	2,719.6
Recurring | Level 2 | Short-term investments — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1.7	6.9
Recurring | Level 2 | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	929.0	1,227.2
Catastrophe Bonds, Fair Value Disclosure	1.0	1.6
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	0.0
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, at fair value	286.8
Total	307.4	473.4
Change in unrealized investment gains (losses)	1.3	(15.7)
Fair Market Value	311.0	489.9
Recurring | Level 3 | Liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities at beginning of year	(3.6)	(16.5)	$ (31.7)
Liability purchases	0.0	0.0
Liability transfers	0.0	0.0
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	12.9	15.2
Change in unrealized gains or losses	12.9	15.2
Recurring | Level 3 | Loss Portfolio Transfer | Liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities at beginning of year	(3.6)	(16.5)	(31.7)
Liability purchases	0.0	0.0
Liability transfers	0.0	0.0
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	12.9	15.2
Change in unrealized gains or losses	12.9	15.2
Recurring | Level 3 | Derivatives at fair value | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0.0	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	(3.6)	(16.5)
Recurring | Level 3 | Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and issuances	71.4	113.7
Transfers Into Level 3	0.0	5.3
Transfers out of Level 3	0.0	12.1
Settlements and sales	(239.7)	(137.2)
Increase (decrease) in fair value included in net income	(10.6)	(12.8)
Fair Market Value		489.9	533.0
Recurring | Level 3 | Fixed income securities | Non-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 — 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 | Fixed income securities, trading
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)	0.0	(17.9)
Fair Market Value	79.7	274.9
Recurring | Level 3 | Commercial mortgage loans | Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and issuances	0.5	40.6
Transfers Into Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Settlements and sales	(184.4)	(61.5)
Increase (decrease) in fair value included in net income	(11.3)	(16.3)
Fair Market Value		274.9	312.1
Recurring | Level 3 | Middle market loans and other private debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in unrealized investment gains (losses)	0.0	0.5
Fair Market Value	61.0	84.8
Recurring | Level 3 | Middle market loans and other private debt | Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and issuances	0.4	18.3
Transfers Into Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Settlements and sales	(35.5)	(41.9)
Increase (decrease) in fair value included in net income	0.4	1.5
Fair Market Value		84.8	106.9
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)	0.0	(0.2)
Fair Market Value	18.5	14.4
Recurring | Level 3 | Global corporate securities | Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and issuances	4.4	0.0
Transfers Into Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Settlements and sales	(0.3)	(0.4)
Increase (decrease) in fair value included in net income	0.0	(0.2)
Fair Market Value		14.4	15.0
Recurring | Level 3 | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in unrealized investment gains (losses)		0.0
Fair Market Value		0.0
Recurring | Level 3 | Equity Securities | Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and issuances		0.0
Transfers Into Level 3		0.0
Transfers out of Level 3		(6.6)
Settlements and sales		0.0
Increase (decrease) in fair value included in net income		0.0
Fair Market Value			6.6
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
Fair Market Value	0.0	18.0
Recurring | Level 3 | Short-term investments | Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and issuances	0.0	18.2
Transfers Into Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Settlements and sales	(7.1)	(25.8)
Increase (decrease) in fair value included in net income	0.0	0.0
Fair Market Value		18.0	25.6
Recurring | Level 3 | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in unrealized investment gains (losses)	1.1	1.7
Fair Market Value	127.6	82.9
Recurring | Level 3 | Asset-backed | Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and issuances	56.1	21.9
Transfers Into Level 3	0.0	5.3
Transfers out of Level 3	0.0	(5.5)
Settlements and sales	(12.4)	(7.6)
Increase (decrease) in fair value included in net income	1.0	2.0
Fair Market Value		82.9	66.8
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	24.2
U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value	1,480.6	1,202.6
Fair Market Value	261.3	245.5
U.S. government | Recurring | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,480.6	1,202.6
U.S. government | Recurring | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	261.3	245.5
U.S. government | Recurring | Level 1 | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,480.6	1,202.6
U.S. government | Recurring | Level 1 | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	261.3	245.5
U.S. government | Recurring | Level 2 | Fixed income securities
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 | Fixed income securities, trading
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 | Fixed income securities
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 | Fixed income securities, trading
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	7.2
U.S. agency | Recurring | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	7.2	7.2
U.S. agency | Recurring | Level 1 | Fixed income securities
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 | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	7.2	7.2
U.S. agency | Recurring | Level 3 | Fixed income securities
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	82.3	128.1
Fair Market Value	1.6	3.1
Municipal | Recurring | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	82.3	128.1
Municipal | Recurring | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1.6	3.1
Municipal | Recurring | Level 1 | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Municipal | Recurring | Level 1 | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Municipal | Recurring | Level 2 | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	82.3	128.1
Municipal | Recurring | Level 2 | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1.6	3.1
Municipal | Recurring | Level 3 | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Municipal | Recurring | Level 3 | Fixed income securities, trading
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	1,986.4	1,959.3
Fair Market Value	151.1	171.5
Corporate | Recurring | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,986.4	1,959.3
Corporate | Recurring | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	151.1	171.5
Corporate | Recurring | Level 1 | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Corporate | Recurring | Level 1 | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Corporate | Recurring | Level 2 | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,986.4	1,959.3
Corporate | Recurring | Level 2 | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	151.1	171.5
Corporate | Recurring | Level 3 | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Corporate | Recurring | Level 3 | Fixed income securities, trading
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	131.3	100.7
Fair Market Value	2.8	8.3
Non-U.S. government-backed corporate | Recurring | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	131.3	100.7
Non-U.S. government-backed corporate | Recurring | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2.8	8.3
Non-U.S. government-backed corporate | Recurring | Level 1 | Fixed income securities
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 | Fixed income securities, trading
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 | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	131.3	100.7
Non-U.S. government-backed corporate | Recurring | Level 2 | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2.8	8.3
Non-U.S. government-backed corporate | Recurring | Level 3 | Fixed income securities
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 | Fixed income securities, trading
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	246.8	273.8
Fair Market Value	24.4	34.8
Non-U.S. government | Recurring | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	246.8	273.8
Non-U.S. government | Recurring | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	24.4	34.8
Non-U.S. government | Recurring | Level 1 | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	195.1	200.4
Non-U.S. government | Recurring | Level 1 | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	9.6	13.0
Non-U.S. government | Recurring | Level 2 | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	51.7	73.4
Non-U.S. government | Recurring | Level 2 | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	14.8	21.8
Non-U.S. government | Recurring | Level 3 | Fixed income securities
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 | Fixed income securities, trading
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]
Fair Market Value	234.5
Fair Market Value	625.2	908.2
Asset-backed | Recurring | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	234.5
Asset-backed | Recurring | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	625.2	908.2
Asset-backed | Recurring | Level 1 | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0
Asset-backed | Recurring | Level 1 | Fixed income securities, trading
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 | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	234.5
Asset-backed | Recurring | Level 2 | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	625.2	908.2
Asset-backed | Recurring | Level 3 | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0
Asset-backed | Recurring | Level 3 | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Non-agency commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value	4.4	5.8
Non-agency commercial mortgage-backed securities | Recurring | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		5.8
Non-agency commercial mortgage-backed securities | Recurring | Level 1 | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0.0
Non-agency commercial mortgage-backed securities | Recurring | Level 2 | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		5.8
Non-agency commercial mortgage-backed securities | Recurring | Level 3 | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0.0
Agency residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value	518.7	445.1
Agency residential mortgage-backed securities | Recurring | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	518.7
Agency residential mortgage-backed securities | Recurring | Level 1 | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0
Agency residential mortgage-backed securities | Recurring | Level 2 | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	518.7
Agency residential mortgage-backed securities | Recurring | Level 3 | Fixed income securities
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	31.1	22.2
Agency mortgage-backed | Recurring | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		445.1
Agency mortgage-backed | Recurring | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	31.1	22.2
Agency mortgage-backed | Recurring | Level 1 | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0.0
Agency mortgage-backed | Recurring | Level 1 | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
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Agency mortgage-backed | Recurring | Level 2 | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		445.1
Agency mortgage-backed | Recurring | Level 2 | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	31.1	22.2
Agency mortgage-backed | Recurring | Level 3 | Fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0.0
Agency mortgage-backed | Recurring | Level 3 | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
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High yield loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
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High yield loans | Recurring | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	102.4	92.1
High yield loans | Recurring | Level 1 | Fixed income securities, trading
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 | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	102.4	92.1
High yield loans | Recurring | Level 3 | Fixed income securities, trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Fixed income securities, trading | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments, available for sale, at fair value		14.9
Privately-held Investments, at fair value		475.0
Fixed income securities, trading | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments, available for sale, at fair value		0.0
Privately-held Investments, at fair value		0.0
Fixed income securities, trading | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments, available for sale, at fair value		0.0
Privately-held Investments, at fair value		0.0
Fixed income securities, trading | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments, available for sale, at fair value		14.9
Privately-held Investments, at fair value		475.0
Short-term investments trading at fair value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		2.1
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		0.2
Short-term investments trading at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
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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	24.2	14.9
Change in unrealized investment gains (losses)	0.2	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		14.9	$ 0.0
Purchases and issuances	10.0	14.7
Transfers Into Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Settlements and sales	0.0	0.0
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