Parent Company Only Financials Balance Sheet (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
ASSETS
Cash and due from banks	$ 444,113	$ 455,482
Investment securities available for sale, at fair value	5,396,812	5,250,585
Other assets	1,010,253	1,006,697
Total assets	26,821,774	24,226,920
LIABILITIES AND STOCKHOLDERS' EQUITY
Commercial paper	74,297	65,484
Senior notes, at par	680,000	585,000
Subordinated debt, at par	250,000	0
Total long-term funding	3,930,117	3,087,267
Accrued expenses and other liabilities	222,285	193,800
Total liabilities	24,021,523	21,335,630
Preferred equity	59,727	61,862
Common stock	1,665	1,750
Total Stockholders' equity	2,800,251	2,891,290	2,936,399	2,865,794
Total liabilities and stockholders' equity	26,821,774	24,226,920
Parent Company [Member]
ASSETS
Cash and due from banks	523,103	148,327
Investment securities available for sale, at fair value	76,159	163,679
Notes receivable from subsidiaries	0	46,594
Investment in subsidiaries	3,166,080	3,161,153
Other assets	94,332	81,529
Total assets	3,859,674	3,601,282
LIABILITIES AND STOCKHOLDERS' EQUITY
Commercial paper	74,297	65,484
Senior notes, at par	680,000	585,000
Subordinated debt, at par	250,000	0
Long-term funding capitalized costs	(143)	1,941
Total long-term funding	929,857	586,941
Accrued expenses and other liabilities	55,269	57,567
Total liabilities	1,059,423	709,992
Preferred equity	59,727	61,862
Common stock	2,740,524	2,829,428
Total Stockholders' equity	2,800,251	2,891,290
Total liabilities and stockholders' equity	$ 3,859,674	$ 3,601,282