Schedule II - Condensed Financial Information of Registrant (Condensed Balance Sheets) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Cash and cash equivalents	$ 1,434,045	$ 791,266	$ 397,749	$ 701,514
Deferred income taxes	38,147	168,578
Other assets	254,127	173,726
Total assets	62,030,736	56,053,472
Liabilities:
Notes and loan payable	494,093	493,755
Federal income tax payable	34,285	0
Other liabilities	1,984,079	1,090,896
Total liabilities	59,180,579	53,761,877
Stockholders' equity:
Common stock	89,331	88,001
Additional paid-in capital	791,446	770,344
Accumulated other comprehensive income	724,599	339,966
Retained earnings	1,244,781	1,093,284
Total stockholders' equity	2,850,157	2,291,595	1,944,535	2,139,876
Total liabilities and stockholders' equity	62,030,736	56,053,472
Parent Company [Member]
Assets
Cash and cash equivalents	22,486	36,394	$ 38,903	$ 61,139
Equity securities of subsidiary trusts	7,429	7,422
Receivable from subsidiaries	166	182
Deferred income taxes	7,945	9,528
Federal income tax recoverable, including amount from subsidiaries	1,059	0
Other assets	1,566	2,540
Total assets, excluding investment in and advances to subsidiaries	40,651	56,066
Investment in and advances to subsidiaries	3,550,405	2,992,217
Total assets	3,591,056	3,048,283
Liabilities:
Notes and loan payable	494,093	493,755
Subordinated debentures payable to subsidiary trusts	242,565	241,853
Federal income tax payable	0	3,614
Other liabilities	4,241	17,466
Total liabilities	740,899	756,688
Stockholders' equity:
Common stock	89,331	88,001
Additional paid-in capital	791,446	770,344
Accumulated other comprehensive income	724,599	339,966
Retained earnings	1,244,781	1,093,284
Total stockholders' equity	2,850,157	2,291,595
Total liabilities and stockholders' equity	$ 3,591,056	$ 3,048,283