Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Other Investments	$ 391,217	$ 431,630
Credit valuation adjustment included in fair value of credit derivative liabilities	138	60
Fair value of derivative liabilities	70,505
Derivative liabilities	$ 76,999	$ 83,409
Weighted average discounted rate of estimated future premium payments to be paid by the VIEs	2.70%	2.50%
Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Percentage of investment portfolio valued using dealer quotes	8.00%	6.00%
Asset-backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Coupon rate	5.97%	5.97%
European ABS Transactions [Member] \| Level 3 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fair value of consolidated VIE debt obligations valued using unobservable inputs by asset class	$ 217,092	$ 2,757,688
Minimum [Member] \| Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Percentage of investment portfolio valued using internal valuation models	2.00%	15.00%
Maximum [Member] \| Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Percentage of investment portfolio valued using external pricing services	90.00%	79.00%
Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fair value of derivative liabilities	$ 0
Derivative liabilities	$ 1,712,062	$ 2,205,264
Weighted average discounted rate of estimated future premium payments to be paid by the VIEs	3.10%	3.10%
Variable Interest Entity, Primary Beneficiary [Member] \| European ABS Transactions [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Coupon rate	2.20%	0.40%
Real Estate [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Other Investments	$ 16,123	$ 33,154
Hedge Funds, Multi-strategy [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Other Investments	0	53,054
Interest Rate Contract [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Other Investments	177,357	136,603
Illiquid Investments [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Other Investments	84,297	67,787
Other Credit Derivatives [Member] \| Level 3 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Derivative liabilities	$ 1,459	$ 566

Weighted average discounted rate of estimated future premium payments to be paid by the VIEs.

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Fair Value of consolidated VIE debt obligations valued using unobservable inputs by asset class

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Investment portfolio valued using dealer quotes.

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