Derivative Instruments - Schedule of Information Regarding Company's Exposure to Credit Loss on the Options Holds (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Credit Derivatives [Line Items]
Options Fair Value	$ 231,044	$ 148,006
Collateral Held	232,159	148,023
Collateral Amounts used to Offset Exposure	230,033	146,864
Excess Collateral	2,126	1,159
Exposure Net of Collateral	1,011	1,142
Barclays | Moody's, Baa3 Rating | S&P BBB Rating
Credit Derivatives [Line Items]
Options Fair Value	53,790	38,905
Collateral Held	53,393	39,104
Collateral Amounts used to Offset Exposure	53,393	38,905
Excess Collateral	0	199
Exposure Net of Collateral	397	0
Credit Suisse | Moody's, Baa2 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Options Fair Value	1,223
Collateral Held	1,150
Collateral Amounts used to Offset Exposure	1,150
Excess Collateral	0
Exposure Net of Collateral	73
Goldman-Sachs | Moody A3 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Options Fair Value	909	615
Collateral Held	670	670
Collateral Amounts used to Offset Exposure	670	615
Excess Collateral	0	55
Exposure Net of Collateral	239	0
ING | Moody Baa1 Rating | S&P A Minus Rating
Credit Derivatives [Line Items]
Options Fair Value	28,382	24,183
Collateral Held	28,830	23,983
Collateral Amounts used to Offset Exposure	28,382	23,983
Excess Collateral	448	0
Exposure Net of Collateral	0	200
Morgan Stanley | Moody A3 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Options Fair Value	26,538	11,649
Collateral Held	26,516	11,059
Collateral Amounts used to Offset Exposure	26,516	11,059
Excess Collateral	0	0
Exposure Net of Collateral	22	590
NATIXIS | Moody's, A1 Rating | S&P A Plus Rating
Credit Derivatives [Line Items]
Options Fair Value	38,289	26,786
Collateral Held	38,700	27,610
Collateral Amounts used to Offset Exposure	38,289	26,786
Excess Collateral	411	824
Exposure Net of Collateral	0	0
SunTrust | Moody Baa1 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Options Fair Value	48,347	23,488
Collateral Held	49,350	23,545
Collateral Amounts used to Offset Exposure	48,320	23,464
Excess Collateral	1,030	81
Exposure Net of Collateral	27	24
Wells Fargo | Moody A2 Rating | S&P A Minus Rating
Credit Derivatives [Line Items]
Options Fair Value	33,566	22,380
Collateral Held	33,550	22,052
Collateral Amounts used to Offset Exposure	33,313	22,052
Excess Collateral	237	0
Exposure Net of Collateral	253	328
Collateral Held in Cash
Credit Derivatives [Line Items]
Collateral Held	147,159	62,899
Collateral Held in Cash | Barclays | Moody's, Baa3 Rating | S&P BBB Rating
Credit Derivatives [Line Items]
Collateral Held	25,393	11,063
Collateral Held in Cash | Credit Suisse | Moody's, Baa2 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Collateral Held	1,150
Collateral Held in Cash | Goldman-Sachs | Moody A3 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Collateral Held	670	670
Collateral Held in Cash | ING | Moody Baa1 Rating | S&P A Minus Rating
Credit Derivatives [Line Items]
Collateral Held	12,830	7,960
Collateral Held in Cash | Morgan Stanley | Moody A3 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Collateral Held	17,516	2,046
Collateral Held in Cash | NATIXIS | Moody's, A1 Rating | S&P A Plus Rating
Credit Derivatives [Line Items]
Collateral Held	38,700	27,610
Collateral Held in Cash | SunTrust | Moody Baa1 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Collateral Held	32,350	6,520
Collateral Held in Cash | Wells Fargo | Moody A2 Rating | S&P A Minus Rating
Credit Derivatives [Line Items]
Collateral Held	18,550	7,030
Collateral Held in Invested Assets
Credit Derivatives [Line Items]
Collateral Held	85,000	85,124
Collateral Held in Invested Assets | Barclays | Moody's, Baa3 Rating | S&P BBB Rating
Credit Derivatives [Line Items]
Collateral Held	28,000	28,041
Collateral Held in Invested Assets | Credit Suisse | Moody's, 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'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	17,025
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