Schedule II - Condensed Financial Information of Registrant (Statements of Income (Parent Company)) (Details) - 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 Income Statements, Captions [Line Items]
Other income									$ 805	$ 376	$ 337
Total revenues	$ 1,934,956	$ 2,031,342	$ 1,848,049	$ 1,870,418	$ 1,971,333	$ 2,019,727	$ 1,855,914	$ 1,807,211	7,684,764	7,654,184	7,206,457
Operating costs and expense									6,911,994	6,757,746	6,474,427
Income before income taxes									772,770	896,438	732,030
Federal income taxes:
Net expense									(219,433)	(292,953)	(227,923)
Net income to common stockholders	154,589	$ 162,054	$ 109,004	$ 123,447	$ 152,790	$ 220,650	$ 108,967	$ 119,511	549,094	601,916	503,694
Dividends from subsidiaries									694,462	700,664	642,421
Parent Company
Condensed Income Statements, Captions [Line Items]
Management fees and investment income including dividends from subsidiaries of $694,462, $700,664 and $642,421 for the years ended December 31, 2017, 2016 and 2015, respectively									738,923	726,742	655,318
Net investment (losses) gains									(4,286)	909	696
Other income									805	376	348
Total revenues									735,442	728,027	656,362
Operating costs and expense									182,145	171,967	143,391
Interest expense									146,929	139,216	128,248
Income before income taxes									406,368	416,844	384,723
Federal income taxes:
Federal income taxes provided by subsidiaries on a separate return basis									115,597	327,520	272,180
Federal income tax expense on a consolidated return basis									(195,261)	(246,389)	(199,322)
Net expense									(79,664)	81,131	72,858
Income before undistributed equity in net income of subsidiaries									326,704	497,975	457,581
Equity in undistributed net income of subsidiaries									222,390	103,941	46,113
Net income to common stockholders	$ 549,094								$ 549,094	$ 601,916	$ 503,694