Loans (Details) - Loan Portfolio - USD ($) $ in Thousands		Sep. 30, 2015	Dec. 31, 2014
Loans and Leases Receivable Disclosure [Abstract]
Commercial and industrial		$ 2,392,860	$ 2,253,556
Agricultural		393,732	358,249
Commercial real estate:
Office, retail, and industrial		1,414,077	1,478,379
Multi-family		539,308	564,421
Construction		192,086	204,236
Other commercial real estate		869,748	887,897
Total commercial real estate		3,015,219	3,134,933
Total corporate loans		5,801,811	5,746,738
Home equity		647,223	543,185
1-4 family mortgages		294,261	291,463
Installment		131,185	76,032
Total consumer loans		1,072,669	910,680
Total loans, excluding covered loans		6,874,480	6,657,418
Covered loans	[1]	51,219	79,435
Total Loans		6,925,699	6,736,853
Deferred loan fees included in total loans		3,846	3,922
Overdrawn demand deposits included in total loans		$ 4,962	$ 3,438

[1] For information on covered loans, see Note 6, "Acquired and Covered Loans."