Net Investment Income By Source (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Investment Income [Line Items]
Gross investment income (loss)	$ 114,570	$ 118,167	$ 116,838
Less investment expenses	(7,043)	(7,018)	(4,654)
Net investment income (loss)	107,527	111,149	112,184
Limited partnership
Net Investment Income [Line Items]
Gross investment income (loss)	3,274	(106)	(778)
Fixed maturity AFS securities
Net Investment Income [Line Items]
Gross investment income (loss)	62,606	67,573	70,495
Equity AFS securities
Net Investment Income [Line Items]
Gross investment income (loss)	1,303	1,962	2,061
Mortgage loans on real estate
Net Investment Income [Line Items]
Gross investment income (loss)	4,665	5,135	4,111
Policy loans
Net Investment Income [Line Items]
Gross investment income (loss)	32,909	34,037	35,435
Derivatives
Net Investment Income [Line Items]
Gross investment income (loss)	6,445	6,818	4,631
Cash and Cash Equivalents
Net Investment Income [Line Items]
Gross investment income (loss)	3,067	2,373	649
Other
Net Investment Income [Line Items]
Gross investment income (loss)	$ 301	$ 375	$ 234