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
Mar. 31, 2019	Mar. 31, 2018
Embedded Derivative
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 596,075	$ 512,526
Total realized and unrealized investment gains (losses) included in other comprehensive income	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liabilities Net Fair Value Change Included In Realized Gains Losses	0	0
Net gain for derivatives included in net investment income	0	0
Net change included in interest credited	58,156	(13,436)
Purchases	0	0
Sales	0	0
Settlements or maturities	0	0
Premiums less benefits	14,254	36,551
Gross transfers into Level 3		0
Gross transfers out of Level 3		0
Ending balance	668,485	535,641
Investment Securities
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	4,346	0
Total realized and unrealized investment gains (losses) included in other comprehensive income	0	0
Net gain for derivatives included in net investment income	0	0
Net change included in interest credited	0	0
Purchases	0	0
Sales	0	0
Settlements or maturities	0	0
Premiums less benefits	0	0
Gross transfers into Level 3		0
Gross transfers out of Level 3		0
Ending balance	4,346	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Assets Net Fair Value Change Included In Realized Gains Losses	0	0
Equity-Indexed Options
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	148,006	220,190
Total realized and unrealized investment gains (losses) included in other comprehensive income	0	0
Net gain for derivatives included in net investment income	66,485	(14,145)
Net change included in interest credited	0	0
Purchases	17,356	16,928
Sales	0	0
Settlements or maturities	(15,691)	(18,665)
Premiums less benefits	0	0
Gross transfers into Level 3		0
Gross transfers out of Level 3		0
Ending balance	216,156	204,308
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Assets Net Fair Value Change Included In Realized Gains Losses	$ 0	$ 0

Fair value assets and liabilities measured on recurring basis unobservable input reconciliation.

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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 assets net change included in interest credited.

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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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- 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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- Definition

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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