Schedule II - Condensed Financial Information of Registrant (Condensed Balance Sheets) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Assets
Cash and cash equivalents	$ 397,749	$ 701,514	$ 897,529	$ 1,268,545
Other assets, including 2015 notes hedges	123,942	280,396
Total assets	49,041,163	43,989,734
Liabilities:
Notes payable	400,000	421,679
Other liabilities, including 2015 notes embedded derivative	629,897	1,009,361
Total liabilities	47,096,628	41,849,858
Stockholders' equity:
Common stock	81,354	76,062
Additional paid-in capital	630,367	513,218
Accumulated other comprehensive income	201,663	721,401
Retained earnings	1,031,151	829,195
Total stockholders' equity	1,944,535	2,139,876	1,384,687	1,720,237
Total liabilities and stockholders' equity	49,041,163	43,989,734
Parent Company [Member]
Assets
Cash and cash equivalents	38,903	61,139	$ 252,541	$ 11,220
Equity securities of subsidiary trusts	7,415	7,409
Receivable from subsidiaries	207	221
Deferred income taxes	11,645	20,612
Federal income tax recoverable, including amount from subsidiaries	7,747	10,430
Other assets, including 2015 notes hedges	14,041	47,308
Total assets, excluding investment in and advances to subsidiaries	79,958	147,119
Investment in and advances to subsidiaries	2,526,972	2,708,085
Total assets	2,606,930	2,855,204
Liabilities:
Notes payable	400,000	421,679
Subordinated debentures payable to subsidiary trusts	246,450	246,243
Other liabilities, including 2015 notes embedded derivative	15,945	47,406
Total liabilities	662,395	715,328
Stockholders' equity:
Common stock	81,354	76,062
Additional paid-in capital	630,367	513,218
Accumulated other comprehensive income	201,663	721,401
Retained earnings	1,031,151	829,195
Total stockholders' equity	1,944,535	2,139,876
Total liabilities and stockholders' equity	$ 2,606,930	$ 2,855,204