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Fair Value Measurements - Financial Assets Measured on Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	$ 4,555.8	$ 4,122.6		$ 3,788.6
Short-term investments, available for sale	66.5	93.6		52.0
Privately-held Investments, available for sale	25.3	14.9		0.0
Fixed income securities, trading at fair value	1,281.3	1,485.7		1,475.5
Short-term investments, trading at fair value	3.2	2.1		6.3
Privately-held Investments, trading	341.2	475.0		533.0
Catastrophe bonds, trading at fair value	1.0	1.6		2.9
Catastrophe bonds, amortized cost	1.0	1.6		5.1
Other investments, at fair value	204.7	209.3		221.3
Derivatives at fair value	26.0	31.7		56.2
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
High yield 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)	107.0	92.1		88.3
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	1,540.6
Short-term investments, available for sale	64.0
Privately-held Investments, available for sale	0.0
Fixed income securities, trading at fair value	277.4
Short-term investments, trading at fair value	3.2
Privately-held Investments, trading	0.0
Catastrophe bonds, trading at fair value	0.0
Other investments, at fair value	0.0
Derivatives at fair value	0.0
Level 1 | High yield 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)	0.0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	3,015.2
Short-term investments, available for sale	2.5
Privately-held Investments, available for sale	0.0
Fixed income securities, trading at fair value	1,003.9
Short-term investments, trading at fair value	0.0
Privately-held Investments, trading	0.0
Catastrophe bonds, trading at fair value	1.0
Other investments, at fair value	0.0
Derivatives at fair value	26.0
Level 2 | High yield 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)	107.0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	0.0
Short-term investments, available for sale	0.0
Privately-held Investments, available for sale	25.3
Fixed income securities, trading at fair value	0.0
Short-term investments, trading at fair value	0.0
Privately-held Investments, trading	341.2
Catastrophe bonds, trading at fair value	0.0
Other investments, at fair value	0.0
Derivatives at fair value	0.0
Level 3 | High yield 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)	0.0
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	115.0	117.1		134.6
Privately-held Investments trading, at fair value		475.0
Fair Value	6,497.3	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	(3.1)	(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	(4.6)	(16.5)		(31.7)
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, amortized cost		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		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
Fair Value	1,885.2	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		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		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		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, amortized cost		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		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
Fair Value	4,045.5	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	(3.1)	(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			0.0
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, amortized cost		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		0.0
Recurring | Level 3
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)	366.5	489.9	$ 480.9	533.0
Privately-held Investments trading, at fair value		475.0
Fair Value	361.9	473.4		501.3
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		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		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	(4.6)	(16.5)		(31.7)
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, amortized cost		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 | 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		14.9
U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	1,326.4	1,202.6		953.0
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	267.1	245.5		261.6
U.S. government | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	1,326.4
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	267.1
U.S. government | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	0.0
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	0.0
U.S. government | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	0.0
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	0.0
U.S. government | Recurring | Fixed Income Securities
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 | Fixed Income Securities
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 | 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 | 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 | 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 | 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	7.4	7.2		8.8
U.S. agency | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	0.0
U.S. agency | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	7.4
U.S. agency | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	0.0
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		8.8
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		8.8
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	79.4	128.1		149.5
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	2.0	3.1		3.6
Municipal | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	0.0
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	0.0
Municipal | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	79.4
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	2.0
Municipal | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	0.0
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	0.0
Municipal | Recurring | Fixed Income Securities
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 | 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 | 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 | Fixed Income Securities
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 | 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 | 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	1,964.2	1,959.3		1,845.0
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	152.6	171.5		162.1
Corporate | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	0.0
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	0.0
Corporate | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	1,964.2
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	152.6
Corporate | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	0.0
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	0.0
Corporate | Recurring | Fixed Income Securities
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 | 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 | 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 | Fixed Income Securities
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 | 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 | 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	144.2	100.7		110.4
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	6.3	8.3		11.6
Non-U.S. government-backed corporate | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	0.0
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	0.0
Non-U.S. government-backed corporate | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	144.2
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	6.3
Non-U.S. government-backed corporate | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	0.0
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	0.0
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		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 | 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 | 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 | Fixed Income Securities
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 | 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 | 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	272.9	273.8		213.6
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	24.2	34.8		30.4
Non-U.S government | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	214.2
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	10.3
Non-U.S government | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	58.7
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	13.9
Non-U.S government | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	0.0
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	0.0
Non-U.S government | Recurring | Fixed Income Securities
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 | Fixed Income Securities
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 | Fixed Income Securities
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 | 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 | 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]
Fair market value	200.1
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	688.7	908.2		896.5
Asset-backed | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	0.0
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	0.0
Asset-backed | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	200.1
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	688.7
Asset-backed | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	0.0
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	0.0
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 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)	33.4	22.2		21.4
Agency mortgage-backed | Level 1
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)	0.0
Agency mortgage-backed | Level 2
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)	33.4
Agency mortgage-backed | Level 3
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)	0.0
Agency mortgage-backed | Recurring | Fixed Income Securities
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 | 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 | 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 | Fixed Income Securities
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 | 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 | 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
Agency commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	4.5	5.8		5.6
Agency commercial mortgage-backed securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	0.0
Agency commercial mortgage-backed securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	4.5
Agency commercial mortgage-backed securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	0.0
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				5.6
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
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				5.6
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
Residential Mortgage-Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	556.7	445.1
Residential Mortgage-Backed Securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	0.0
Residential Mortgage-Backed Securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	556.7
Residential Mortgage-Backed Securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	$ 0.0
Residential Mortgage-Backed Securities | Recurring | Fixed Income Securities
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 | Fixed Income Securities
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 | Fixed Income Securities
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 | Fixed Income Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		$ 0.0

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