Schedule II - Condensed Financial Information of Registrant (Balance Sheets (Parent Company)) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 30, 2011
Assets:
Cash and cash equivalents	$ 817,602	$ 950,471	$ 795,285	$ 763,631
Fixed maturity securities available for sale at fair value (cost $1,317,058 and $1,059,834 at December 31, 2018 and 2017, respectively)	13,606,812	13,551,250
Equity securities, at fair value (cost $3,430 in 2018 and 2017)	279,006	576,647
Current federal income taxes	703	0
Deferred federal income taxes	35,490	0
Property, furniture and equipment at cost, less accumulated depreciation	416,372	422,960
Other assets	501,413	434,554
Total assets	24,895,977	24,299,917
Liabilities:
Other liabilities	1,005,184	981,987
Current federal income taxes	0	11,327
Deferred federal income taxes	0	86,764
Subordinated debentures	907,491	728,218
Total liabilities	19,416,179	18,848,754
Stockholders' equity:
Preferred stock	0	0
Common stock	47,024	47,024
Additional paid-in capital	1,063,144	1,048,283
Retained earnings (including accumulated undistributed net income of subsidiaries of $5,068,139 and $5,073,268 at December 31, 2018 and 2017, respectively)	7,558,619	6,956,882
Accumulated other comprehensive (loss) income	(510,470)	68,541
Treasury stock, at cost	(2,720,466)	(2,709,386)
Total common stockholders’ equity	5,437,851	5,411,344
Total liabilities and equity	24,895,977	24,299,917
Fixed maturity securities available for sale at cost	13,596,057	13,242,580
Parent Company
Assets:
Cash and cash equivalents	83,950	45,062	$ 124,803		$ 195,658
Fixed maturity securities available for sale at fair value (cost $1,317,058 and $1,059,834 at December 31, 2018 and 2017, respectively)	1,307,347	1,052,240
Loans receivable	51,544	53,019
Equity securities, at fair value (cost $3,430 in 2018 and 2017)	3,430	3,430
Investment in subsidiaries	6,786,999	7,140,108
Current federal income taxes	9,068	0
Deferred federal income taxes	66,995	0
Property, furniture and equipment at cost, less accumulated depreciation	13,391	14,421
Other assets	12,340	10,819
Total assets	8,335,064	8,319,099
Liabilities:
Due to subsidiaries	116,125	232,756
Other liabilities	115,562	128,002
Current federal income taxes	0	10,486
Deferred federal income taxes	0	51,757
Subordinated debentures	907,491	728,218
Senior notes	1,758,035	1,756,536
Total liabilities	2,897,213	2,907,755
Stockholders' equity:
Preferred stock	0	0
Common stock	47,024	47,024
Additional paid-in capital	1,063,144	1,048,283
Retained earnings (including accumulated undistributed net income of subsidiaries of $5,068,139 and $5,073,268 at December 31, 2018 and 2017, respectively)	7,558,619	6,956,882
Accumulated other comprehensive (loss) income	(510,470)	68,541
Treasury stock, at cost	(2,720,466)	(2,709,386)
Total common stockholders’ equity	5,437,851	5,411,344
Total liabilities and equity	8,335,064	8,319,099
Fixed maturity securities available for sale at cost	1,317,058	1,059,834
Equity securities available for sale at cost	3,430	3,430
Accumulated undistributed net income of subsidiaries	$ 5,068,139	$ 5,073,268