Loans (Details) - Loan Portfolio - USD ($) $ in Thousands		Jun. 30, 2015	Dec. 31, 2014
Loans and Leases Receivable Disclosure [Abstract]
Commercial and industrial		$ 2,366,056	$ 2,253,556
Agricultural		377,410	358,249
Commercial real estate:
Office, retail, and industrial		1,432,502	1,478,379
Multi-family		557,947	564,421
Construction		190,970	204,236
Other commercial real estate		871,119	887,897
Total commercial real estate		3,052,538	3,134,933
Total corporate loans		5,796,004	5,746,738
Home equity		599,320	543,185
1-4 family mortgages		283,562	291,463
Installment		113,382	76,032
Total consumer loans		996,264	910,680
Total loans, excluding covered loans		6,792,268	6,657,418
Covered loans	[1]	57,917	79,435
Total Loans		6,850,185	6,736,853
Deferred loan fees included in total loans		4,360	3,922
Overdrawn demand deposits included in total loans		$ 2,841	$ 3,438

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