Loans (Allowance for credit losses on financing receivables) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	$ 6,436		$ 4,755
Ending Balance Under ASC 450-20	54,888		50,931
Total	61,324	$ 58,656	55,686	$ 54,051	$ 54,999	$ 54,324
Ending Balance Evaluated for Impairment Under ASC 310-10-35	82,461		78,468
Ending Balance Evaluated for Impairment Under ASC 450-20	7,396,980		6,314,758
Total	7,479,441		6,393,226
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	5,721,138		4,809,875
Commercial | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	2,821		1,613
Ending Balance Under ASC 450-20	17,888		16,485
Total	20,709	19,395	18,098	17,168	16,180	14,765
Ending Balance Evaluated for Impairment Under ASC 310-10-35	14,072		7,415
Ending Balance Evaluated for Impairment Under ASC 450-20	2,020,168		1,639,191
Total	2,034,240		1,646,606
Commercial | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	212		766
Ending Balance Under ASC 450-20	23,515		21,184
Total	23,727	23,469	21,950	21,861	23,797	24,319
Ending Balance Evaluated for Impairment Under ASC 310-10-35	13,164		23,705
Ending Balance Evaluated for Impairment Under ASC 450-20	3,673,734		3,139,564
Total	3,686,898		3,163,269
Agricultural and agricultural real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	1,648		546
Ending Balance Under ASC 450-20	4,061		3,712
Total	5,709	4,716	4,258	3,832	3,983	4,210
Ending Balance Evaluated for Impairment Under ASC 310-10-35	20,887		13,304
Ending Balance Evaluated for Impairment Under ASC 450-20	541,466		498,284
Total	562,353		511,588
Residential real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	310		430
Ending Balance Under ASC 450-20	1,547		1,794
Total	1,857	2,141	2,224	2,263	2,183	2,263
Ending Balance Evaluated for Impairment Under ASC 310-10-35	25,481		27,141
Ending Balance Evaluated for Impairment Under ASC 450-20	657,570		597,138
Total	683,051		624,279
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	1,445		1,400
Ending Balance Under ASC 450-20	7,877		7,756
Total	9,322	$ 8,935	9,156	$ 8,927	$ 8,856	$ 8,767
Ending Balance Evaluated for Impairment Under ASC 310-10-35	8,857		6,903
Ending Balance Evaluated for Impairment Under ASC 450-20	504,042		440,581
Total	$ 512,899		$ 447,484