Fair Value Measurements - Additional Information (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities	$ 283	$ 19
Derivative liabilities	$ 126,000	$ 90,000
Weighted average discounted rate of estimated future premium payments to be paid by the VIEs	2.20%	2.40%
Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Derivative liabilities	$ 1,771,000	$ 1,657,000
Weighted average discounted rate of estimated future premium payments to be paid by the VIEs	3.30%	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	$ 391,000	$ 296,000
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%	4.00%
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%	94.00%