Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities		$ 75	$ 138
Other investments	$ 469,438		391,217
Derivative liabilities	$ 88,484		$ 76,999
Weighted average discounted rate of estimated future premium payments to be paid by the VIEs	2.50%		2.70%
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	$ 296,874		$ 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		2.80%	3.10%
Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Derivative liabilities	$ 1,818,273		$ 1,712,062