Schedule II - Condensed Financial Information of Registrant (Balance Sheets (Parent Company)) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2012	Dec. 30, 2011
Assets:
Cash and cash equivalents	$ 795,285	$ 763,631	$ 674,441	$ 839,738
Fixed maturity securities available for sale at fair value (cost $899,206 and $201,256 at December 31, 2016 and 2015, respectively)	13,190,668	12,444,394
Loans and Leases Receivable, Gross	23,419	0
Equity securities available for sale, at fair value (cost $3,430 in 2016 and 2015)	669,200	150,866
Current federal income taxes	14,768	55,763
Property, furniture and equipment at cost, less accumulated depreciation	349,432	348,224
Other assets	402,550	442,782
Total assets	23,364,844	21,730,967
Liabilities:
Other liabilities	916,318	837,937
Deferred federal income taxes	134,365	6,811
Subordinated debentures	727,630	340,320
Total liabilities	18,283,710	17,097,759
Stockholders' equity:
Preferred stock	0	0
Common stock	47,024	47,024
Additional paid-in capital	1,037,446	1,005,455
Retained earnings (including accumulated undistributed net income of subsidiaries of $4,850,878 and $4,746,934 at December 31, 2016 and 2015, respectively)	6,595,987	6,178,070
Accumulated other comprehensive income (loss)	55,568	(66,698)
Treasury stock, at cost	(2,688,817)	(2,563,605)
Total common stockholders’ equity	5,047,208	4,600,246
Total liabilities and equity	23,364,844	21,730,967
Fixed maturity securities available for sale at cost	12,869,809	12,012,041
Equity securities available for sale at cost	220,587	165,192
Parent Company
Assets:
Cash and cash equivalents	124,803	195,658	$ 90,693		$ 17,315
Fixed maturity securities available for sale at fair value (cost $899,206 and $201,256 at December 31, 2016 and 2015, respectively)	894,748	201,738
Equity securities available for sale, at fair value (cost $3,430 in 2016 and 2015)	3,430	3,430
Investment in subsidiaries	6,891,246	6,454,065
Deferred federal income taxes	0	37,135
Current federal income taxes	15,455	51,512
Property, furniture and equipment at cost, less accumulated depreciation	14,798	13,150
Other assets	7,122	6,153
Total assets	7,975,021	6,962,841
Liabilities:
Due to subsidiaries	234,014	143,669
Other liabilities	120,160	115,737
Current federal income taxes	90,966
Deferred federal income taxes		0
Subordinated debentures	727,630	340,320
Senior notes	1,755,043	1,762,869
Total liabilities	2,927,813	2,362,595
Stockholders' equity:
Preferred stock	0	0
Common stock	47,024	47,024
Additional paid-in capital	1,037,446	1,005,455
Retained earnings (including accumulated undistributed net income of subsidiaries of $4,850,878 and $4,746,934 at December 31, 2016 and 2015, respectively)	6,595,987	6,178,070
Accumulated other comprehensive income (loss)	55,568	(66,698)
Treasury stock, at cost	(2,688,817)	(2,563,605)
Total common stockholders’ equity	5,047,208	4,600,246
Total liabilities and equity	7,975,021	6,962,841
Fixed maturity securities available for sale at cost		201,256
Equity securities available for sale at cost		3,430
Accumulated undistributed net income of subsidiaries		$ 4,746,934