Investments - Schedule of Analysis of Investment Operations (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014		Dec. 31, 2013
Net Investment Income [Line Items]
Investment income, gross									$ 786,542	$ 769,531		$ 744,969
Less investment expense									(12,591)	(11,245)		(10,319)
Net investment income	$ 194,319	$ 193,213	$ 194,823	$ 191,596	$ 190,717	$ 189,588	$ 189,930	$ 188,051	773,951	758,286	[1]	734,650	[1]
Fixed Maturities [Member]
Net Investment Income [Line Items]
Investment income, gross									747,663	732,925		709,756
Equity Securities [Member]
Net Investment Income [Line Items]
Investment income, gross									13	8		323
Policy Loans [Member]
Net Investment Income [Line Items]
Investment income, gross									36,763	35,015		33,471
Other Long-term Investments [Member]
Net Investment Income [Line Items]
Investment income, gross									2,008	1,508		1,281
Short-term Investments [Member]
Net Investment Income [Line Items]
Investment income, gross									$ 95	$ 75		$ 138

[1] Certain balances were adjusted to give effect to discontinued operations and the adoption of new accounting guidance as described in Note 1- Significant Accounting Policies under the caption "Low-Income Housing Tax Credit Interests."