Derivative Instruments - Schedule of Derivative Instruments Reported in Financial Position (Detail) - Derivatives Not Designated as Hedging Instruments $ in Thousands	Dec. 31, 2018 USD ($) derivative	Dec. 31, 2017 USD ($) derivative
Equity-Indexed Options \| Other Invested Assets
Derivatives, Fair Value [Line Items]
Number of Instruments \| derivative	493	468
Notional Amount, asset	$ 2,391,000	$ 1,885,600
Estimated Fair Value, assets	$ 148,006	$ 220,190
Embedded Derivative \| Policyholders' Account Balances
Derivatives, Fair Value [Line Items]
Number of Instruments \| derivative	90,440	76,621
Notional Amount, liability	$ 2,327,769	$ 1,819,523
Estimated Fair Value, liabilities	$ 596,075	$ 512,526

Nominal or face amount used to calculate payments on the derivative asset.

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