Short-Term Borrowings and Long-Term Debt Short Term (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Short-term Debt [Line Items]
Maximum borrowing capacity	$ 1,200,000,000
Short-term borrowings	329,719,000	1,258,629,000	868,399,000
Collateralized borrowings availability at discount window	1,100,000,000	2,000,000,000
Federal funds purchased
Short-term Debt [Line Items]
Short-term borrowings	6,219,000	582,436,000	592,470,000
Maximum month-end outstanding amount	577,581,000	848,179,000	636,562,000
Short-term FHLB advances
Short-term Debt [Line Items]
Short-term borrowings	70,000,000	400,000,000	0
Maximum month-end outstanding amount	600,000,000	600,000,000	25,000,000
Customer repurchase agreements
Short-term Debt [Line Items]
Short-term borrowings	158,394,000	175,621,000	156,238,000
Maximum month-end outstanding amount	244,729,000	215,305,000	258,734,000
Customer short-term promissory notes
Short-term Debt [Line Items]
Short-term borrowings	95,106,000	100,572,000	119,691,000
Maximum month-end outstanding amount	$ 95,106,000	$ 115,129,000	$ 152,570,000