Net Investment Income and Realized Investment Gains (Losses) - Summary of Realized Investment Gains (Losses) and Other Than Temporary Impairment Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	$ 7,528	$ (649)	$ 63,598	$ 27,548
Other-than-temporary impairment losses	(19,407)	(1,608)	(22,904)	(3,045)
Bonds Available-for-Sale [Member]
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	(34)	1,925	10,063	21,837
Other-than-temporary impairment losses	(286)		(286)	(41)
Equity Securities [Member]
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	7,669	229	44,579	10,293
Other-than-temporary impairment losses	(19,121)	(1,608)	(22,618)	(3,004)
Mortgage Loans [Member]
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	639	(1,551)	(94)	(5,424)
Real Estate [Member]
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	(739)	(1,242)	9,094	1,787
Other Invested Assets [Member]
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	$ (7)	$ (10)	$ (44)	$ (945)