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Fair Value Measurements - Financial Assets Measured on Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe Bonds, Fair Value Disclosure	$ 2.4	$ 5.1
Derivative assets	17.2	56.2
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	6,245.6	6,100.9
Recurring | Derivatives at fair value | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	17.2	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	(13.4)	(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	(23.4)	(31.7)
Recurring | Total fixed income securities — Available for sale | Non-agency commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	5.7
Recurring | Fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	3,908.2	3,788.6
Recurring | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	169.3	52.0
Recurring | Fixed income securities, trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,478.5	1,475.5
Privately-held Investments, at fair value	482.4	533.0
Catastrophe Bonds, Fair Value Disclosure	2.4	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	11.0	6.3
Recurring | Real estate fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	213.4	221.3
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	1,632.5	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]
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 | Total fixed income securities — Available for sale | 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	1,206.5	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	169.2	51.9
Recurring | Level 1 | Fixed income securities, trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	247.7	272.2
Privately-held Investments, at fair value	0.0	0.0
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	9.1	6.3
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 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	3,940.7	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]
Derivative assets	17.2	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	(13.4)	(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 | Total fixed income securities — Available for sale | Non-agency commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	5.7
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	2,701.7	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	0.1	0.1
Recurring | Level 2 | Fixed income securities, trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,230.8	1,203.3
Privately-held Investments, at fair value	0.0	0.0
Catastrophe Bonds, Fair Value Disclosure	2.4	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	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 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	459.0	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]
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	(23.4)	(31.7)
Recurring | Level 3 | Total fixed income securities — Available for sale | 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
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, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.0	0.0
Privately-held Investments, at fair value	482.4	533.0
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	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
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	1,013.8	953.0
U.S. government | Recurring | Fixed income securities, trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	235.0	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	1,013.8	953.0
U.S. government | Recurring | Level 1 | Fixed income securities, trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	235.0	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 | Fixed income securities, trading, 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 | Fixed income securities, trading, 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 | Recurring | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	8.2	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	8.2	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 | Recurring | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	149.5	149.5
Municipal | Recurring | Fixed income securities, trading, 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 | Fixed income securities, trading, 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	149.5	149.5
Municipal | Recurring | Level 2 | Fixed income securities, trading, 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 | Fixed income securities, trading, 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 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,875.9	1,845.0
Corporate | Recurring | Fixed income securities, trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	165.4	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 | Fixed income securities, trading, 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	1,875.9	1,845.0
Corporate | Recurring | Level 2 | Fixed income securities, trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	165.4	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 | Fixed income securities, trading, 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 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	110.9	110.4
Non-U.S. government-backed corporate | Recurring | Fixed income securities, trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	9.6	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 | Fixed income securities, trading, 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	110.9	110.4
Non-U.S. government-backed corporate | Recurring | Level 2 | Fixed income securities, trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	9.6	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 | Fixed income securities, trading, 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 | Recurring | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	269.2	213.6
Non-U.S government | Recurring | Fixed income securities, trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	34.1	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	192.7	145.3
Non-U.S government | Recurring | Level 1 | Fixed income securities, trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	12.7	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	76.5	68.3
Non-U.S government | Recurring | Level 2 | Fixed income securities, trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	21.4	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 | Fixed income securities, trading, 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 | Fixed income securities, trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	924.6	896.5
Asset-backed | Recurring | Level 1 | Fixed income securities, trading, 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 | Fixed income securities, trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	924.6	896.5
Asset-backed | Recurring | Level 3 | Fixed income securities, trading, at fair value
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 | 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
Non-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
Non-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
Non-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
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	475.0	502.7
Agency mortgage-backed | Recurring | Fixed income securities, trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	20.1	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	0.0
Agency mortgage-backed | Recurring | Level 1 | Fixed income securities, trading, 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	475.0	502.7
Agency mortgage-backed | Recurring | Level 2 | Fixed income securities, trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	20.1	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	0.0
Agency mortgage-backed | Recurring | Level 3 | Fixed income securities, trading, 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 | Fixed income securities, trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	86.6	88.3
High yield loans | Recurring | Level 1 | Fixed income securities, trading, 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 | Fixed income securities, trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	86.6	88.3
High yield loans | Recurring | Level 3 | Fixed income securities, trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	$ 0.0	$ 0.0

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Catastrophe Bonds, Fair Value Disclosure

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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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