Schedule II - Condensed Financial Information of Registrant (Statements of Income (Parent Company)) (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Jun. 30, 2016	Jun. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Income Statements, Captions [Line Items]
Other income									$ 376	$ 337	$ 1,308
Total revenues	$ 1,971,333	$ 1,807,211	$ 1,811,056	$ 1,744,679	$ 1,855,914	$ 1,789,765	$ 2,019,727	$ 1,860,957	7,654,184	7,206,457	7,128,928
Operating costs and expense									6,757,746	6,474,427	6,176,732
Income before income taxes									896,438	732,030	952,196
Federal income taxes:
Net expense	302,593								(292,953)	(227,923)	(302,593)
Equity in undistributed net income of subsidiaries									103,944
Net income to common stockholders	152,790	$ 119,511	$ 109,745	$ 118,307	$ 108,967	$ 123,035	$ 220,650	$ 152,607	601,916	503,694	648,884
Dividends from subsidiaries									642,421	503,483	269,626
Parent Company
Condensed Income Statements, Captions [Line Items]
Management fees and investment income including dividends from subsidiaries of $700,664, $642,421 and $503,483 for the years ended December 31, 2016, 2015 and 2014, respectively									726,742	655,318	515,775
Net investment gains									909	696	5,487
Other income									376	348	450
Total revenues									728,027	656,362	521,712
Operating costs and expense									171,967	143,391	148,288
Interest expense									139,216	128,248	125,352
Income before income taxes									416,844	384,723	248,072
Federal income taxes:
Federal income taxes provided by subsidiaries on a separate return basis									327,520	272,180	366,721
Federal income tax expense on a consolidated return basis									(246,389)	(199,322)	(273,310)
Net expense									81,131	72,858	93,411
Income before undistributed equity in net income of subsidiaries									497,975	457,581	341,483
Equity in undistributed net income of subsidiaries									103,941	46,113	307,401
Net income to common stockholders	$ 601,916								$ 601,919	$ 503,694	$ 648,884