Derivative Instruments - Schedule of Derivative Instruments Reported in Financial Position (Detail) - Derivatives Not Designated as Hedging Instruments $ in Thousands	Sep. 30, 2019 USD ($) derivative	Dec. 31, 2018 USD ($) derivative
Equity-indexed options | Other invested assets
Derivatives, Fair Value [Line Items]
Number of Instruments | derivative	483	493
Notional Amount, assets	$ 2,587,000	$ 2,391,000
Estimated Fair Value, assets	$ 224,184	$ 148,006
Equity-indexed embedded derivative | Policyholders’ account balances
Derivatives, Fair Value [Line Items]
Number of Instruments | derivative	99,767	90,440
Notional Amount, liabilities	$ 2,484,968	$ 2,327,769
Estimated Fair Value, liabilities	$ 691,766	$ 596,075