Loans (Allowance for credit losses on financing receivables) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	$ 5,471	$ 4,755
Ending Balance Under ASC 450-20	53,185	50,931
Total	58,656	55,686	$ 54,999	$ 54,324
Ending Balance Evaluated for Impairment Under ASC 310-10-35	83,572	78,468
Ending Balance Evaluated for Impairment Under ASC 450-20	6,664,245	6,314,758
Total	6,747,817	6,393,226
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	5,129,777	4,809,875
Commercial | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	2,425	1,613
Ending Balance Under ASC 450-20	16,970	16,485
Total	19,395	18,098	16,180	14,765
Ending Balance Evaluated for Impairment Under ASC 310-10-35	9,005	7,415
Ending Balance Evaluated for Impairment Under ASC 450-20	1,797,678	1,639,191
Total	1,806,683	1,646,606
Commercial | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	736	766
Ending Balance Under ASC 450-20	22,733	21,184
Total	23,469	21,950	23,797	24,319
Ending Balance Evaluated for Impairment Under ASC 310-10-35	22,920	23,705
Ending Balance Evaluated for Impairment Under ASC 450-20	3,300,174	3,139,564
Total	3,323,094	3,163,269
Agricultural and agricultural real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	787	546
Ending Balance Under ASC 450-20	3,929	3,712
Total	4,716	4,258	3,983	4,210
Ending Balance Evaluated for Impairment Under ASC 310-10-35	16,896	13,304
Ending Balance Evaluated for Impairment Under ASC 450-20	501,490	498,284
Total	518,386	511,588
Residential real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	386	430
Ending Balance Under ASC 450-20	1,755	1,794
Total	2,141	2,224	2,183	2,263
Ending Balance Evaluated for Impairment Under ASC 310-10-35	28,324	27,141
Ending Balance Evaluated for Impairment Under ASC 450-20	596,401	597,138
Total	624,725	624,279
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	1,137	1,400
Ending Balance Under ASC 450-20	7,798	7,756
Total	8,935	9,156	$ 8,856	$ 8,767
Ending Balance Evaluated for Impairment Under ASC 310-10-35	6,427	6,903
Ending Balance Evaluated for Impairment Under ASC 450-20	468,502	440,581
Total	$ 474,929	$ 447,484