Investments in Securities - Change in Net Unrealized Gains (Losses) on Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Schedule of Available-for-sale Securities [Line Items]
Change in net unrealized gains (losses) on securities during the year			$ (217,164)	$ 161,912
Adjustments for
Deferred policy acquisition costs			30,491	(17,175)
Participating policyholders' interest			7,222	(8,526)
Deferred federal income tax benefit (expense)			62,248	(47,160)
Change in net unrealized gains (losses) on securities, net of tax	$ (70,900)	$ (17,708)	(117,203)	89,051
Bonds Available-for-Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Change in net unrealized gains (losses) on securities during the year			(81,173)	82,701
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Change in net unrealized gains (losses) on securities during the year			$ (135,991)	$ 79,211