Derivative Instruments - Schedule of Derivative Instruments Reported in Financial Position (Detail) - Derivatives Not Designated as Hedging Instruments $ in Thousands	Mar. 31, 2019 USD ($) derivative	Dec. 31, 2018 USD ($) derivative
Equity-Indexed Options | Other Invested Assets
Derivatives, Fair Value [Line Items]
Number of Instruments | derivative	486	493
Notional Amount, assets	$ 2,413,900	$ 2,391,000
Estimated Fair Value, assets	$ 216,156	$ 148,006
Embedded Derivative | Policyholders' Account Balances
Derivatives, Fair Value [Line Items]
Number of Instruments | derivative	92,783	90,440
Notional Amount, liabilities	$ 2,383,124	$ 2,327,769
Estimated Fair Value, liabilities	$ 668,485	$ 596,075