Derivative Instruments - Schedule of Derivative Instruments Reported in Financial Position (Detail) - Derivatives Not Designated as Hedging Instruments $ in Thousands	Jun. 30, 2019 USD ($) derivative	Dec. 31, 2018 USD ($) derivative
Equity-indexed options | Other invested assets
Derivatives, Fair Value [Line Items]
Number of Instruments | derivative	484	493
Notional Amount, assets	$ 2,498,744	$ 2,391,000
Estimated Fair Value, assets	$ 231,044	$ 148,006
Equity-indexed embedded derivative | Policyholders’ account balances
Derivatives, Fair Value [Line Items]
Number of Instruments | derivative	96,014	90,440
Notional Amount, liabilities	$ 2,455,001	$ 2,327,769
Estimated Fair Value, liabilities	$ 695,676	$ 596,075