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Fair Value Measurements - Additional Information (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018	Sep. 30, 2019 USD ($)	Sep. 30, 2018	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fair Value Assets, Level 1 to Level 2 Transfers, Number	0	0	0	0
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities					$ 35	$ 138
Other investments	$ 462,427		$ 462,427			391,217
Derivative liabilities	$ 103,382		$ 103,382			$ 76,999
Weighted average discounted rate of estimated future premium payments to be paid by the VIEs	2.40%		2.40%			2.70%
Fair Value Assets, Level 2 to Level 1 Transfers, Number	0	0	0	0
Fair Value Assets, Level 2 to Level 3 Transfers, Number	0	0	0	0
Fair Value Assets, Level 3 to Level 2 Transfers, Number	0	0	0	0
Fair Value Measured at Net Asset Value Per Share [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Other investments	$ 287,748		$ 287,748			$ 279,941
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					4.00%	8.00%
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%	2.00%
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					94.00%	90.00%
Variable Interest Entity [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Weighted average discounted rate of estimated future premium payments to be paid by the VIEs					2.60%	3.10%
Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Derivative liabilities	$ 1,779,077		$ 1,779,077			$ 1,712,062

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Investment portfolio valued using dealer quotes.

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