Fair Value of Financial Instruments - Financial Instruments Measured at Fair Value on Recurring Basis Using (Level 3) Inputs (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Embedded Derivative
Fair Value, Liabilities Measured on Recurring Basis [Roll Forward]
Beginning balance	$ 695,676	$ 592,913	$ 596,075	$ 512,526
Net gain for derivatives included in net investment income	0	0	0	0
Net change included in interest credited	11,462	52,797	105,873	56,960
Purchases, sales and settlements or maturities
Purchases	0	0	0	0
Sales	0		0
Settlements or maturities	0	0	0	0
Premiums less benefits	(15,372)	6,426	(10,182)	82,650
Ending balance	691,766	652,136	691,766	652,136
Investment Securities
Fair Value, Assets Measured on Recurring Basis [Roll Forward]
Beginning balance	4,346	0	4,346	0
Net gain for derivatives included in net investment income	0	0	0	0
Net change included in interest credited	0	0	0	0
Purchases, sales and settlements or maturities
Purchases	463	4,233	463	4,233
Sales	113		113
Settlements or maturities	0	0	0	0
Premiums less benefits	0	0	0	0
Ending balance	4,696	4,233	4,696	4,233
Equity-Indexed Options
Fair Value, Assets Measured on Recurring Basis [Roll Forward]
Beginning balance	231,044	217,341	148,006	220,190
Net gain for derivatives included in net investment income	6,278	50,866	95,888	58,433
Net change included in interest credited	0	0	0	0
Purchases, sales and settlements or maturities
Purchases	17,029	15,195	55,797	58,207
Sales	13,397		13,397
Settlements or maturities	(16,770)	(26,433)	(62,110)	(79,861)
Premiums less benefits	0	0	0	0
Ending balance	$ 224,184	$ 256,969	$ 224,184	$ 256,969