Fair Value of Financial Instruments - Financial Instruments Measured at Fair Value on Recurring Basis Using (Level 3) Inputs (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability
Carry value transfers in		$ 0
Embedded Derivative
Liability
Beginning balance	$ 512,526	314,330	$ 242,412
Total realized and unrealized investment gains (losses) included in other comprehensive income		0	0
Net fair value change included in realized gains (losses)		0	0
Net gain (loss) for derivatives included in net investment income	0	0	0
Net change included in interest credited	(17,862)	98,351	25,239
Purchases	0	0	0
Sales	0	0	0
Settlements or maturities	0	0	0
Premiums less benefits	101,411	99,845	46,679
Gross transfers into Level 3		0	0
Gross transfers out of Level 3		0	0
Ending balance	596,075	512,526	314,330
Investment Securities
Assets
Beginning balance	0	14,264	20,130
Total realized and unrealized investment gains (losses) included in other comprehensive income		(4,465)	481
Net fair value change included in realized gains (losses)		0	0
Net gain (loss) for derivatives included in net investment income	0	0	0
Net change included in interest credited	0	0	0
Purchases	4,346	0	0
Sales	0	(12,436)	0
Settlements or maturities	0	(7,020)	(425)
Premiums less benefits	0	0	0
Carry value transfers in		15,000
Gross transfers into Level 3		382	908
Gross transfers out of Level 3		(5,725)	(6,830)
Ending balance	4,346	0	14,264
Equity-Indexed Options
Assets
Beginning balance	220,190	156,479	123,007
Total realized and unrealized investment gains (losses) included in other comprehensive income		0	0
Net fair value change included in realized gains (losses)		0	0
Net gain (loss) for derivatives included in net investment income	(55,093)	90,433	28,400
Net change included in interest credited	0	0	0
Purchases	72,033	47,134	27,961
Sales	(18)	(12,837)	0
Settlements or maturities	(89,106)	(61,019)	(22,889)
Premiums less benefits	0	0	0
Carry value transfers in		0
Gross transfers into Level 3		0	0
Gross transfers out of Level 3		0	0
Ending balance	$ 148,006	$ 220,190	$ 156,479

Fair value measurement with unobservable inputs reconciliation asset premiums less benefits.

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Fair value measurement with unobservable inputs reconciliation liability premiums less benefits losses.

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Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Liability Transfers Into Level 3

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Fair value measurement with unobservable inputs reconciliation recurring basis assets net change included in interest credited.

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Fair value measurement with unobservable inputs reconciliation recurring basis assets net fair value change included in realized gains losses.

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Fair value measurement with unobservable inputs reconciliation recurring basis liabilities net change included in interest credited.

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Fair value measurement with unobservable inputs reconciliation recurring basis liabilities, net fair value change included in realized gains losses.

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Amount of gain (loss) in accumulated other comprehensive income (AOCI) of derivative hedging forecasted acquisition of investment in debt security measured at amortized cost (held-to-maturity), transferred to investment in debt security measured at fair value with changes in fair value recognized in other comprehensive income (available-for-sale) and net income (trading).

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

Amount of transfers of financial instrument classified as a liability into level 3 of the fair value hierarchy.

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Amount of transfers of financial instrument classified as a liability out of level 3 of the fair value hierarchy.

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Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of gain (loss) recognized in other comprehensive income (loss) for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

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Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of gain (loss) recognized on the income statement for financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of gain (loss) recognized in other comprehensive income (loss) for financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of purchases of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of sales of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of settlements of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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