Schedule II - Condensed Financial Information of Registrant (Balance Sheets (Parent Company)) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 30, 2011
Assets:
Cash and cash equivalents	$ 674,441	$ 839,738	$ 905,670	$ 911,742
Fixed maturity securities available for sale at fair value (cost $272,283 and $61,084 at December 31, 2014 and 2013, respectively)	12,705,160	11,616,844
Equity securities available for sale, at fair value (cost $3,738 in 2014 and $4,668 in 2013)	170,991	283,338
Deferred Federal income taxes	0	20,120
Current Federal income taxes	67,623	24,737
Property, furniture and equipment at cost, less accumulated depreciation	332,098	339,448
Other assets	369,558	335,225
Total assets	21,716,691	20,551,796
Liabilities:
Other liabilities	859,736	848,749
Deferred Federal income taxes	37,452	0
Subordinated debentures	340,060	339,800
Total liabilities	17,092,557	16,182,402
Stockholders' equity:
Preferred stock	0	0
Common stock	47,024	47,024
Additional paid-in capital	991,512	967,440
Retained earnings (including accumulated undistributed net income of subsidiaries of $4,700,821 and $4,393,420 at December 31, 2014 and 2013, respectively)	5,732,410	5,265,015
Accumulated other comprehensive income	183,550	189,391
Treasury stock, at cost	(2,364,551)	(2,132,835)
Total common stockholders’ equity	4,589,945	4,336,035
Total liabilities and equity	21,716,691	20,551,796
Equity securities available for sale at cost	160,295	203,627
Parent Company
Assets:
Cash and cash equivalents	90,693	17,315	162,972		270,637
Fixed maturity securities available for sale at fair value (cost $272,283 and $61,084 at December 31, 2014 and 2013, respectively)	273,773	63,115
Equity securities available for sale, at fair value (cost $3,738 in 2014 and $4,668 in 2013)	3,738	11,240
Investment in subsidiaries	6,693,731	6,250,979
Due from subsidiaries	0	13,648
Deferred Federal income taxes	0	51,346
Current Federal income taxes	62,882	17,540
Property, furniture and equipment at cost, less accumulated depreciation	9,852	9,028
Other assets	6,978	5,795
Total assets	7,141,647	6,440,006
Liabilities:
Due to subsidiaries	87,540	0
Other liabilities	162,648	148,903
Current Federal income taxes	149
Deferred Federal income taxes		0
Subordinated debentures	340,060	339,800
Senior notes	1,961,305	1,615,268
Total liabilities	2,551,702	2,103,971
Stockholders' equity:
Preferred stock	0	0
Common stock	47,024	47,024
Additional paid-in capital	991,512	967,440
Retained earnings (including accumulated undistributed net income of subsidiaries of $4,700,821 and $4,393,420 at December 31, 2014 and 2013, respectively)	5,732,410	5,265,015
Accumulated other comprehensive income	183,550	189,391
Treasury stock, at cost	(2,364,551)	(2,132,835)
Total common stockholders’ equity	4,589,945	4,336,035
Total liabilities and equity	7,141,647	6,440,006
Fixed maturity securities available for sale at cost	272,283	61,084
Equity securities available for sale at cost	3,738	4,668
Accumulated undistributed net income of subsidiaries	$ 4,700,821	$ 4,393,420