Derivative Instruments - Schedule of Derivative Instruments Reported in Financial Position (Detail) $ in Thousands	Sep. 30, 2015 USD ($) Derivative	Dec. 31, 2014 USD ($) Derivative
Derivatives, Fair Value [Line Items]
Estimated Fair Value of other invested assets	$ 141,385	$ 189,449
Derivative Financial Instruments, Estimated Fair Value	$ 207,302	$ 208,187
Derivatives Not Designated as Hedging Instruments [Member] | Other Invested Assets [Member] | Equity-Indexed Options [Member]
Derivatives, Fair Value [Line Items]
Number of Instruments | Derivative	454	436
Derivative Notional Amount, Asset	$ 1,249,300	$ 1,095,300
Estimated Fair Value of other invested assets	$ 141,385	$ 189,449
Policyholders' Account Balances [Member] | Derivatives Not Designated as Hedging Instruments [Member] | Equity-Indexed Embedded Derivative [Member]
Derivatives, Fair Value [Line Items]
Number of Instruments | Derivative	48,923	42,287
Derivative Notional Amount, Liability	$ 1,005,100	$ 961,300
Derivative Financial Instruments, Estimated Fair Value	$ 207,302	$ 208,187