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		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Embedded Derivative
Fair Value, Liabilities Measured on Recurring Basis [Roll Forward]
Beginning balance	$ 668,485	$ 535,641	$ 596,075	$ 512,526
Net gain for derivatives included in net investment income	0	0	0	0
Net change included in interest credited	36,255	17,599	94,411	4,163
Purchases, sales and settlements or maturities
Purchases	0	0	0	0
Settlements or maturities	0	0	0	0
Premiums less benefits	(9,064)	39,673	5,190	76,224
Ending balance	695,676	592,913	695,676	592,913
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	0	0	0	0
Settlements or maturities	0	0	0	0
Premiums less benefits	0	0	0	0
Ending balance	4,346	0	4,346	0
Equity-Indexed Options
Fair Value, Assets Measured on Recurring Basis [Roll Forward]
Beginning balance	216,156	204,308	148,006	220,190
Net gain for derivatives included in net investment income	23,125	21,712	89,610	7,567
Net change included in interest credited	0	0	0	0
Purchases, sales and settlements or maturities
Purchases	21,412	26,084	38,768	43,012
Settlements or maturities	(29,649)	(34,763)	(45,340)	(53,428)
Premiums less benefits	0	0	0	0
Ending balance	$ 231,044	$ 217,341	$ 231,044	$ 217,341