Schedule II - Condensed Financial Information of Registrant (Balance Sheets (Parent Company)) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 30, 2011
Assets:
Cash and cash equivalents	$ 950,471	$ 795,285	$ 763,631	$ 674,441
Fixed maturity securities available for sale at fair value (cost $1,059,834 and $899,206 at December 31, 2017 and 2016, respectively)	13,551,250	13,190,668
Equity securities available for sale, at fair value (cost $3,430 in 2017 and 2016)	576,647	669,200
Current federal income taxes	0	14,768
Property, furniture and equipment at cost, less accumulated depreciation	422,960	349,432
Other assets	434,554	402,550
Total assets	24,299,917	23,364,844
Liabilities:
Other liabilities	981,987	916,318
Current federal income taxes	11,327	0
Deferred federal income taxes	86,764	134,365
Subordinated debentures	728,218	727,630
Total liabilities	18,848,754	18,283,710
Stockholders' equity:
Preferred stock	0	0
Common stock	47,024	47,024
Additional paid-in capital	1,048,283	1,037,446
Retained earnings (including accumulated undistributed net income of subsidiaries of $5,073,268 and $4,850,878 at December 31, 2017 and 2016, respectively)	6,956,882	6,595,987
Accumulated other comprehensive income	68,541	55,568
Treasury stock, at cost	(2,709,386)	(2,688,817)
Total common stockholders’ equity	5,411,344	5,047,208
Total liabilities and equity	24,299,917	23,364,844
Fixed maturity securities available for sale at cost	13,242,580	12,869,809
Equity securities available for sale at cost	206,005	220,587
Parent Company
Assets:
Cash and cash equivalents	45,062	124,803	$ 195,658		$ 90,693
Fixed maturity securities available for sale at fair value (cost $1,059,834 and $899,206 at December 31, 2017 and 2016, respectively)	1,052,240	894,748
Loans receivable	53,019	23,419
Equity securities available for sale, at fair value (cost $3,430 in 2017 and 2016)	3,430	3,430
Investment in subsidiaries	7,140,108	6,891,246
Current federal income taxes	0	15,455
Property, furniture and equipment at cost, less accumulated depreciation	14,421	14,798
Other assets	10,819	7,122
Total assets	8,319,099	7,975,021
Liabilities:
Due to subsidiaries	232,756	234,014
Other liabilities	128,002	120,160
Current federal income taxes	10,486	0
Deferred federal income taxes	51,757	90,966
Subordinated debentures	728,218	727,630
Senior notes	1,756,536	1,755,043
Total liabilities	2,907,755	2,927,813
Stockholders' equity:
Preferred stock	0	0
Common stock	47,024	47,024
Additional paid-in capital	1,048,283	1,037,446
Retained earnings (including accumulated undistributed net income of subsidiaries of $5,073,268 and $4,850,878 at December 31, 2017 and 2016, respectively)	6,956,882	6,595,987
Accumulated other comprehensive income	68,541	55,568
Treasury stock, at cost	(2,709,386)	(2,688,817)
Total common stockholders’ equity	5,411,344	5,047,208
Total liabilities and equity	8,319,099	7,975,021
Fixed maturity securities available for sale at cost	1,059,834	899,206
Equity securities available for sale at cost	3,430	3,430
Accumulated undistributed net income of subsidiaries	$ 5,073,268	$ 4,850,878