Net Investment Income and Realized Investment Gains (Losses) - Summary of Net Investment Income (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Net Investment Income [Line Items]
Net Investment Income, Total	$ 184,482	$ 236,489	$ 597,357	$ 697,604
Bonds Available-for-Sale [Member]
Net Investment Income [Line Items]
Net Investment Income, Total	138,707	148,715	421,543	450,110
Equity Securities [Member]
Net Investment Income [Line Items]
Net Investment Income, Total	9,353	8,146	27,869	26,488
Mortgage Loans [Member]
Net Investment Income [Line Items]
Net Investment Income, Total	50,681	51,652	149,682	159,010
Real Estate [Member]
Net Investment Income [Line Items]
Net Investment Income, Total	13,555	14,245	11,949	11,347
Options [Member]
Net Investment Income [Line Items]
Net Investment Income, Total	(33,682)	6,562	(34,646)	29,011
Other Invested Assets [Member]
Net Investment Income [Line Items]
Net Investment Income, Total	$ 5,868	$ 7,169	$ 20,960	$ 21,638