Derivative Instruments - Schedule of Information Regarding Company's Exposure to Credit Loss on the Options Holds (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Credit Derivatives [Line Items]
Options fair value	$ 256,005	$ 148,006
Collateral held	257,559	148,023
Collateral amounts used to offset exposure	253,997	146,864
Excess collateral	3,562	1,159
Exposure net of collateral	2,008	1,142
Barclays | Moody Baa3 Rating | S&P BBB Rating
Credit Derivatives [Line Items]
Options fair value	54,583	38,905
Collateral held	55,343	39,104
Collateral amounts used to offset exposure	54,583	38,905
Excess collateral	760	199
Exposure net of collateral	0	0
Credit Suisse | Moody Baa2 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Options fair value	7,117
Collateral held	7,390
Collateral amounts used to offset exposure	7,009
Excess collateral	381
Exposure net of collateral	108
Goldman-Sachs | Moody A3 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Options fair value	1,053	615
Collateral held	930	670
Collateral amounts used to offset exposure	930	615
Excess collateral	0	55
Exposure net of collateral	123	0
ING | Moody Baa1 Rating | S&P A Minus Rating
Credit Derivatives [Line Items]
Options fair value	30,330	24,183
Collateral held	30,940	23,983
Collateral amounts used to offset exposure	30,330	23,983
Excess collateral	610	0
Exposure net of collateral	0	200
Morgan Stanley | Moody A3 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Options fair value	34,988	11,649
Collateral held	34,926	11,059
Collateral amounts used to offset exposure	34,926	11,059
Excess collateral	0	0
Exposure net of collateral	62	590
NATIXIS | Moody A1 Rating | S&P A Plus Rating
Credit Derivatives [Line Items]
Options fair value	29,918	26,786
Collateral held	30,200	27,610
Collateral amounts used to offset exposure	29,918	26,786
Excess collateral	282	824
Exposure net of collateral	0	0
SunTrust | Moody A3 Rating | S&P A Minus Rating
Credit Derivatives [Line Items]
Options fair value	60,360
Collateral held	58,720
Collateral amounts used to offset exposure	58,645
Excess collateral	75
Exposure net of collateral	1,715
SunTrust | Moody Baa1 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Options fair value		23,488
Collateral held		23,545
Collateral amounts used to offset exposure		23,464
Excess collateral		81
Exposure net of collateral		24
Wells Fargo | Moody A2 Rating | S&P A Minus Rating
Credit Derivatives [Line Items]
Options fair value	37,656	22,380
Collateral held	39,110	22,052
Collateral amounts used to offset exposure	37,656	22,052
Excess collateral	1,454	0
Exposure net of collateral	0	328
Collateral Held in Cash
Credit Derivatives [Line Items]
Collateral held	172,559	62,899
Collateral Held in Cash | Barclays | Moody Baa3 Rating | S&P BBB Rating
Credit Derivatives [Line Items]
Collateral held	27,343	11,063
Collateral Held in Cash | Credit Suisse | Moody Baa2 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Collateral held	7,390
Collateral Held in Cash | Goldman-Sachs | Moody A3 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Collateral held	930	670
Collateral Held in Cash | ING | Moody Baa1 Rating | S&P A Minus Rating
Credit Derivatives [Line Items]
Collateral held	14,940	7,960
Collateral Held in Cash | Morgan Stanley | Moody A3 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Collateral held	25,926	2,046
Collateral Held in Cash | NATIXIS | Moody A1 Rating | S&P A Plus Rating
Credit Derivatives [Line Items]
Collateral held	30,200	27,610
Collateral Held in Cash | SunTrust | Moody A3 Rating | S&P A Minus Rating
Credit Derivatives [Line Items]
Collateral held	41,720
Collateral Held in Cash | SunTrust | Moody Baa1 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Collateral held		6,520
Collateral Held in Cash | Wells Fargo | Moody A2 Rating | S&P A Minus Rating
Credit Derivatives [Line Items]
Collateral held	24,110	7,030
Collateral Held in Invested Assets
Credit Derivatives [Line Items]
Collateral held	85,000	85,124
Collateral Held in Invested Assets | Barclays | Moody Baa3 Rating | S&P BBB Rating
Credit Derivatives [Line Items]
Collateral held	28,000	28,041
Collateral Held in Invested Assets | Credit Suisse | Moody Baa2 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Collateral held	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	16,023
Collateral Held in Invested Assets | Morgan Stanley | Moody A3 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Collateral held	9,000	9,013
Collateral Held in Invested Assets | NATIXIS | Moody A1 Rating | S&P A Plus Rating
Credit Derivatives [Line Items]
Collateral held	0	0
Collateral Held in Invested Assets | SunTrust | Moody A3 Rating | S&P A Minus Rating
Credit Derivatives [Line Items]
Collateral held	17,000
Collateral Held in Invested Assets | SunTrust | Moody Baa1 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Collateral held		17,025
Collateral Held in Invested Assets | Wells Fargo | Moody A2 Rating | S&P A Minus Rating
Credit Derivatives [Line Items]
Collateral held	$ 15,000	$ 15,022