Schedule II. Condensed Financial Information of Registrant (Condensed Statements of Operations) (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net investment income	$ 212,955	$ 213,872	$ 212,776	$ 208,282	$ 205,488	$ 202,720	$ 201,642	$ 197,053	$ 847,885	$ 806,903	$ 773,951
Realized investment gains (losses)	17,469	12,595	(705)	(5,748)	(18,463)	3,482	4,005	293	23,611	(10,683)	(8,791)
Total revenue	1,056,899	1,046,015	1,029,078	1,023,581	975,345	989,773	991,884	977,627	4,155,573	3,934,629	3,766,065
General operating expenses									257,255	232,064	223,858
Interest expense									84,532	83,345	76,642
Total benefits and expenses									3,324,925	3,162,394	2,999,878
Income before income taxes	216,371	220,610	201,926	191,741	175,982	201,461	199,344	195,448	830,648	772,235	766,187
Income taxes									627,615	(232,645)	(249,894)
Net income	$ 1,027,346	$ 153,334	$ 140,273	$ 133,541	$ 135,448	$ 151,869	$ 138,429	$ 124,033	1,454,494	549,779	527,100
Other comprehensive income (loss):
Comprehensive income (loss)									2,048,794	895,406	(238,405)
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net investment income									26,130	25,352	23,715
Realized investment gains (losses)									(2,791)	0	8
Total revenue									23,339	25,352	23,723
General operating expenses									61,447	52,613	54,100
Reimbursements from affiliates									(52,776)	(54,288)	(53,436)
Interest expense									88,474	86,853	79,677
Total benefits and expenses									97,145	85,178	80,341
Income before income taxes									(73,806)	(59,826)	(56,618)
Income taxes									(9,874)	23,479	15,542
Net operating loss before equity in earnings of affiliates									(83,680)	(36,347)	(41,076)
Equity in earnings of affiliates									1,538,174	586,126	568,176
Net income									1,454,494	549,779	527,100
Other comprehensive income (loss):
Attributable to Parent Company									(8,409)	(11,314)	(3,539)
Attributable to affiliates									602,709	356,941	(761,966)
Comprehensive income (loss)									$ 2,048,794	$ 895,406	$ (238,405)