Parent Company Only Financial Information, Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Cash and due from banks	$ 483,666	$ 446,558
Investment securities available for sale, at fair value	4,043,446	4,680,226
Other assets	482,294	421,787
Total assets	30,483,594	29,139,315
Liabilities and stockholders' equity
Commercial paper	67,467	101,688
Senior notes, at par	250,000	250,000
Subordinated notes, at par	250,000	250,000
Other Long-term Debt and Capitalized Costs	2,718	3,393
Total long-term funding	3,397,450	2,761,795
Accrued expenses and other liabilities	318,797	254,622
Total liabilities	27,246,151	26,048,003
Preferred equity	159,929	159,929
Common equity	1,618	1,630
Total stockholders’ equity	3,237,443	3,091,312	$ 2,937,246	$ 2,800,251
Total liabilities and stockholders’ equity	30,483,594	29,139,315
Parent Company Only
Assets
Cash and due from banks	146,877	402,786
Investment securities available for sale, at fair value	11,645	14,389
Notes Receivable, Related Parties, Noncurrent	301,378	0
Investments in and receivable due from subsidiaries	3,320,090	3,256,763
Other assets	51,466	53,242
Total assets	3,831,456	3,727,180
Liabilities and stockholders' equity
Commercial paper	67,467	101,688
Senior notes, at par	250,000	250,000
Subordinated notes, at par	250,000	250,000
Other Long-term Debt and Capitalized Costs	(2,718)	(3,393)
Total long-term funding	497,282	496,607
Accrued expenses and other liabilities	29,264	37,573
Total liabilities	594,013	635,868
Preferred equity	159,929	159,929
Common equity	3,077,514	2,931,383
Total stockholders’ equity	3,237,443	3,091,312
Total liabilities and stockholders’ equity	$ 3,831,456	$ 3,727,180