Loans (Allowance for credit losses on financing receivables) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	$ 6,003		$ 4,755
Ending Balance Under ASC 450-20	55,218		50,931
Total	61,221	$ 61,324	55,686	$ 54,885	$ 54,051	$ 54,324
Ending Balance Evaluated for Impairment Under ASC 310-10-35	87,669		78,468
Ending Balance Evaluated for Impairment Under ASC 450-20	7,280,454		6,314,758
Total	7,368,123		6,393,226
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	5,610,953		4,809,875
Commercial | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	4,165		1,613
Ending Balance Under ASC 450-20	17,904		16,485
Total	22,069	20,709	18,098	16,970	17,168	14,765
Ending Balance Evaluated for Impairment Under ASC 310-10-35	23,414		7,415
Ending Balance Evaluated for Impairment Under ASC 450-20	1,938,808		1,639,191
Total	1,962,222		1,646,606
Commercial | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	214		766
Ending Balance Under ASC 450-20	23,476		21,184
Total	23,690	23,727	21,950	20,540	21,861	24,319
Ending Balance Evaluated for Impairment Under ASC 310-10-35	14,415		23,705
Ending Balance Evaluated for Impairment Under ASC 450-20	3,634,316		3,139,564
Total	3,648,731		3,163,269
Agricultural and agricultural real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	260		546
Ending Balance Under ASC 450-20	4,306		3,712
Total	4,566	5,709	4,258	6,127	3,832	4,210
Ending Balance Evaluated for Impairment Under ASC 310-10-35	17,522		13,304
Ending Balance Evaluated for Impairment Under ASC 450-20	556,526		498,284
Total	574,048		511,588
Residential real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	279		430
Ending Balance Under ASC 450-20	1,571		1,794
Total	1,850	1,857	2,224	2,266	2,263	2,263
Ending Balance Evaluated for Impairment Under ASC 310-10-35	23,647		27,141
Ending Balance Evaluated for Impairment Under ASC 450-20	653,294		597,138
Total	676,941		624,279
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	1,085		1,400
Ending Balance Under ASC 450-20	7,961		7,756
Total	9,046	$ 9,322	9,156	$ 8,982	$ 8,927	$ 8,767
Ending Balance Evaluated for Impairment Under ASC 310-10-35	8,671		6,903
Ending Balance Evaluated for Impairment Under ASC 450-20	497,510		440,581
Total	$ 506,181		$ 447,484