Derivative Instruments - Schedule of Information Regarding Company's Exposure to Credit Loss on the Options Holds (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Credit Derivatives [Line Items]
Options Fair Value	$ 125,988	$ 256,005
Collateral Held	134,479	257,559
Collateral Amounts used to Offset Exposure	125,919	253,997
Excess Collateral	8,560	3,562
Exposure Net of Collateral	69	2,008
Barclays | Moody's, Baa3 Rating | S&P BBB Rating
Credit Derivatives [Line Items]
Options Fair Value	26,494	54,583
Collateral Held	27,913	55,343
Collateral Amounts used to Offset Exposure	26,494	54,583
Excess Collateral	1,419	760
Exposure Net of Collateral	0	0
Credit Suisse | Moody's, Baa2 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Options Fair Value	1,459		$ 7,117
Collateral Held	1,590		7,390
Collateral Amounts used to Offset Exposure	1,459		7,009
Excess Collateral	131		381
Exposure Net of Collateral	0		108
Goldman-Sachs | Moody A3 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Options Fair Value	709	1,053
Collateral Held	640	930
Collateral Amounts used to Offset Exposure	640	930
Excess Collateral	0	0
Exposure Net of Collateral	69	123
ING | Moody Baa1 Rating | S&P A Minus Rating
Credit Derivatives [Line Items]
Options Fair Value	23,193	30,330
Collateral Held	23,560	30,940
Collateral Amounts used to Offset Exposure	23,193	30,330
Excess Collateral	367	610
Exposure Net of Collateral	0	0
Morgan Stanley | Moody A3 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Options Fair Value	10,421	34,988
Collateral Held	12,136	34,926
Collateral Amounts used to Offset Exposure	10,421	34,926
Excess Collateral	1,715	0
Exposure Net of Collateral	0	62
NATIXIS | Moody's, A1 Rating | S&P A Plus Rating
Credit Derivatives [Line Items]
Options Fair Value	19,961	29,918
Collateral Held	21,250	30,200
Collateral Amounts used to Offset Exposure	19,961	29,918
Excess Collateral	1,289	282
Exposure Net of Collateral	0	0
SunTrust | Moody Baa1 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Options Fair Value	25,018	60,360
Collateral Held	27,040	58,720
Collateral Amounts used to Offset Exposure	25,018	58,645
Excess Collateral	2,022	75
Exposure Net of Collateral	0	1,715
Wells Fargo | Moody A2 Rating | S&P A Minus Rating
Credit Derivatives [Line Items]
Options Fair Value	18,733	37,656
Collateral Held	20,350	39,110
Collateral Amounts used to Offset Exposure	18,733	37,656
Excess Collateral	1,617	1,454
Exposure Net of Collateral	0	0
Collateral  Held in Cash
Credit Derivatives [Line Items]
Collateral Held	49,479	172,559
Collateral  Held in Cash | Barclays | Moody's, Baa3 Rating | S&P BBB Rating
Credit Derivatives [Line Items]
Collateral Held	(87)	27,343
Collateral  Held in Cash | Credit Suisse | Moody's, Baa2 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Collateral Held	1,590		7,390
Collateral  Held in Cash | Goldman-Sachs | Moody A3 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Collateral Held	640	930
Collateral  Held in Cash | ING | Moody Baa1 Rating | S&P A Minus Rating
Credit Derivatives [Line Items]
Collateral Held	7,560	14,940
Collateral  Held in Cash | Morgan Stanley | Moody A3 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Collateral Held	3,136	25,926
Collateral  Held in Cash | NATIXIS | Moody's, A1 Rating | S&P A Plus Rating
Credit Derivatives [Line Items]
Collateral Held	21,250	30,200
Collateral  Held in Cash | SunTrust | Moody Baa1 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Collateral Held	10,040	41,720
Collateral  Held in Cash | Wells Fargo | Moody A2 Rating | S&P A Minus Rating
Credit Derivatives [Line Items]
Collateral Held	5,350	24,110
Collateral Held in Invested Assets
Credit Derivatives [Line Items]
Collateral Held	85,000	85,000
Collateral Held in Invested Assets | Barclays | Moody's, Baa3 Rating | S&P BBB Rating
Credit Derivatives [Line Items]
Collateral Held	28,000	28,000
Collateral Held in Invested Assets | Credit Suisse | Moody's, Baa2 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Collateral Held	0		$ 0
Collateral Held in Invested Assets | Goldman-Sachs | Moody A3 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Collateral Held	0	0
Collateral Held in Invested Assets | ING | Moody Baa1 Rating | S&P A Minus Rating
Credit Derivatives [Line Items]
Collateral Held	16,000	16,000
Collateral Held in Invested Assets | Morgan Stanley | Moody A3 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Collateral Held	9,000	9,000
Collateral Held in Invested Assets | NATIXIS | Moody's, A1 Rating | S&P A Plus Rating
Credit Derivatives [Line Items]
Collateral Held	0	0
Collateral Held in Invested Assets | SunTrust | Moody Baa1 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Collateral Held	17,000	17,000
Collateral Held in Invested Assets | Wells Fargo | Moody A2 Rating | S&P A Minus Rating
Credit Derivatives [Line Items]
Collateral Held	$ 15,000	$ 15,000