Schedule II. Condensed Financial Information of Registrant (Condensed Statements of Operations) (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Condensed Financial Statements, Captions [Line Items]
Net investment income	$ 235,071	$ 231,432	$ 231,568	$ 228,991	$ 227,456	$ 228,905	$ 227,425	$ 226,673	$ 927,062	$ 910,459	$ 882,512
Realized gains (losses)	25,015	1,501	(4,790)	(26,097)	2,195	11,943	5,154	1,329	(4,371)	20,621	(1,804)
Total revenue	1,228,941	1,195,042	1,180,884	1,133,054	1,136,576	1,141,279	1,130,461	1,119,216	4,737,921	4,527,532	4,303,751
Other operating expense									301,038	304,825	279,585
Interest expense									86,704	84,306	90,076
Total benefits and expenses									3,841,237	3,596,253	3,440,080
Income before income taxes	249,984	231,538	212,241	202,921	227,088	247,330	228,760	228,101	896,684	931,279	863,671
Income tax benefit (expense)									(164,911)	(170,397)	(162,161)
Net income	$ 204,240	$ 188,945	$ 173,048	$ 165,540	$ 187,061	$ 201,818	$ 186,566	$ 185,345	731,773	760,790	701,466
Other comprehensive income (loss):
Attributable to Parent Company									1,184,414	1,525,355	(1,104,799)
Comprehensive income (loss)									1,916,187	2,286,145	(403,333)
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net investment income									30,199	28,869	28,077
Realized gains (losses)									12,792	0	(11,078)
Total revenue									42,991	28,869	16,999
Other operating expense									57,679	68,419	65,762
Reimbursements from affiliates									(68,556)	(65,928)	(61,620)
Interest expense									90,197	89,317	94,159
Total benefits and expenses									79,320	91,808	98,301
Income before income taxes									(36,329)	(62,939)	(81,302)
Income tax benefit (expense)									7,773	13,133	15,262
Net operating loss before equity in earnings of affiliates									(28,556)	(49,806)	(66,040)
Equity in earnings of affiliates, net of tax									760,329	810,596	767,506
Net income									731,773	760,790	701,466
Other comprehensive income (loss):
Attributable to Parent Company									(21,477)	(11,379)	23,805
Attributable to affiliates									1,205,891	1,536,734	(1,128,604)
Comprehensive income (loss)									$ 1,916,187	$ 2,286,145	$ (403,333)