Schedule II. Condensed Financial Information of Registrant (Condensed Statements of Operations) (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Net investment income	$ 227,456	$ 228,905	$ 227,425	$ 226,673	$ 224,233	$ 221,627	$ 218,568	$ 218,084	$ 910,459	$ 882,512	$ 847,885
Realized gains (losses)	2,195	11,943	5,154	1,329	(16,600)	1,032	11,813	1,951	20,621	(1,804)	23,611
Total revenue	1,136,576	1,141,279	1,130,461	1,119,216	1,064,737	1,083,802	1,084,776	1,070,436	4,527,532	4,303,751	4,155,573
Other operating expense									304,825	279,585	257,255
Interest expense									84,306	90,076	84,532
Total expenses									3,596,253	3,440,080	3,324,925
Income before income taxes	227,088	247,330	228,760	228,101	203,635	220,330	226,864	212,842	931,279	863,671	830,648
Income tax benefit (expense)									(170,397)	(162,161)	627,615
Net income	$ 187,061	$ 201,818	$ 186,566	$ 185,345	$ 164,717	$ 178,724	$ 184,425	$ 173,600	760,790	701,466	1,454,494
Other comprehensive income (loss):
Attributable to Parent Company									1,525,355	(1,104,799)	594,300
Comprehensive income (loss)									2,286,145	(403,333)	2,048,794
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net investment income									28,869	28,077	26,130
Realized gains (losses)									0	(11,078)	(2,791)
Total revenue									28,869	16,999	23,339
Other operating expense									68,419	65,762	61,447
Reimbursements from affiliates									(65,928)	(61,620)	(52,776)
Interest expense									89,317	94,159	88,474
Total expenses									91,808	98,301	97,145
Income before income taxes									(62,939)	(81,302)	(73,806)
Income tax benefit (expense)									13,133	15,262	(9,874)
Net operating loss before equity in earnings of affiliates									(49,806)	(66,040)	(83,680)
Equity in earnings of affiliates, net of tax									810,596	767,506	1,538,174
Net income									760,790	701,466	1,454,494
Other comprehensive income (loss):
Attributable to Parent Company									(11,379)	23,805	(8,409)
Attributable to affiliates									1,536,734	(1,128,604)	602,709
Comprehensive income (loss)									$ 2,286,145	$ (403,333)	$ 2,048,794