Derivative Instruments - Schedule of Information Regarding Company's Exposure to Credit Loss on the Options Holds (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Credit Derivatives [Line Items]
Options Fair Value	$ 224,184	$ 148,006
Collateral Held	223,449	148,023
Collateral Amounts used to Offset Exposure	222,691	146,864
Excess Collateral	758	1,159
Exposure Net of Collateral	1,493	1,142
Barclays | Moody's, Baa3 Rating | S&P BBB Rating
Credit Derivatives [Line Items]
Options Fair Value	52,648	38,905
Collateral Held	52,363	39,104
Collateral Amounts used to Offset Exposure	52,363	38,905
Excess Collateral	0	199
Exposure Net of Collateral	285	0
Credit Suisse | Moody's, Baa2 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Options Fair Value	2,345
Collateral Held	2,110
Collateral Amounts used to Offset Exposure	2,110
Excess Collateral	0
Exposure Net of Collateral	235
Goldman-Sachs | Moody A3 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Options Fair Value	934	615
Collateral Held	930	670
Collateral Amounts used to Offset Exposure	930	615
Excess Collateral	0	55
Exposure Net of Collateral	4	0
ING | Moody Baa1 Rating | S&P A Minus Rating
Credit Derivatives [Line Items]
Options Fair Value	28,587	24,183
Collateral Held	29,110	23,983
Collateral Amounts used to Offset Exposure	28,587	23,983
Excess Collateral	523	0
Exposure Net of Collateral	0	200
Morgan Stanley | Moody A3 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Options Fair Value	27,802	11,649
Collateral Held	27,716	11,059
Collateral Amounts used to Offset Exposure	27,716	11,059
Excess Collateral	0	0
Exposure Net of Collateral	86	590
NATIXIS | Moody's, A1 Rating | S&P A Plus Rating
Credit Derivatives [Line Items]
Options Fair Value	27,069	26,786
Collateral Held	27,160	27,610
Collateral Amounts used to Offset Exposure	27,069	26,786
Excess Collateral	91	824
Exposure Net of Collateral	0	0
SunTrust | Moody Baa1 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Options Fair Value	54,475	23,488
Collateral Held	53,690	23,545
Collateral Amounts used to Offset Exposure	53,669	23,464
Excess Collateral	21	81
Exposure Net of Collateral	806	24
Wells Fargo | Moody A2 Rating | S&P A Minus Rating
Credit Derivatives [Line Items]
Options Fair Value	30,324	22,380
Collateral Held	30,370	22,052
Collateral Amounts used to Offset Exposure	30,247	22,052
Excess Collateral	123	0
Exposure Net of Collateral	77	328
Collateral Held in Cash
Credit Derivatives [Line Items]
Collateral Held	138,449	62,899
Collateral Held in Cash | Barclays | Moody's, Baa3 Rating | S&P BBB Rating
Credit Derivatives [Line Items]
Collateral Held	24,363	11,063
Collateral Held in Cash | Credit Suisse | Moody's, Baa2 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Collateral Held	2,110
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	13,110	7,960
Collateral Held in Cash | Morgan Stanley | Moody A3 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Collateral Held	18,716	2,046
Collateral Held in Cash | NATIXIS | Moody's, A1 Rating | S&P A Plus Rating
Credit Derivatives [Line Items]
Collateral Held	27,160	27,610
Collateral Held in Cash | SunTrust | Moody Baa1 Rating | S&P BBB Plus Rating
Credit Derivatives [Line Items]
Collateral Held	36,690	6,520
Collateral Held in Cash | Wells Fargo | Moody A2 Rating | S&P A Minus Rating
Credit Derivatives [Line Items]
Collateral Held	15,370	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