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Fair Value Measurements - Inputs Used (Details)	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums	$ 3,442,400,000	$ 3,446,900,000	$ 3,360,900,000
Minimum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums		50,100,000
Reserve for losses		4,200,000
Initial value of issuance		325,000,000
Maximum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums		61,100,000
Reserve for losses		61,900,000
Initial value of issuance		325,000,000
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables)	0	(4,600,000)
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan Notes, fair value		4,600,000	$ 86,600,000
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables)	0	(4,600,000)
Silverton 2016 | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan Notes, fair value		$ 4,600,000
Measurement Input, Expected Term | Maximum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contract period		365 days
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, at fair value	279,700,000
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, at fair value	$ 279,700,000	$ 0
Discounted cash flow | Commercial mortgage loans | Minimum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.050
Discounted cash flow | Commercial mortgage loans | Maximum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.063
Discounted cash flow | Commercial mortgage loans | Weighted Average | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.058
Discounted cash flow | Commercial mortgage loans | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments, at fair value	$ 125,700,000
Discounted cash flow | Middle market loans | Minimum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.068
Discounted cash flow | Middle market loans | Maximum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.103
Discounted cash flow | Middle market loans | Weighted Average | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.079
Discounted cash flow | Middle market loans | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments, at fair value	$ 111,700,000
Discounted cash flow | Asset-backed | Minimum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.064
Discounted cash flow | Asset-backed | Maximum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.064
Discounted cash flow | Asset-backed | Weighted Average | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.064
Discounted cash flow | Asset-backed | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments, at fair value	$ 8,700,000
Transaction value | Commercial mortgage loans | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments, at fair value	30,900,000
Transaction value | Equity securities | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments, at fair value	$ 2,700,000

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Amount of gain (loss) recognized in income from asset and liability measured at fair value on recurring basis using unobservable input (level 3).

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Amount of direct and assumed premiums written.

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