Loans (Allowance for credit losses on financing receivables) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	$ 6,948		$ 6,753
Ending Balance Under ASC 450-20	47,103		47,571
Total	54,051	$ 54,999	54,324	$ 51,756	$ 49,738	$ 48,685
Ending Balance Evaluated for Impairment Under ASC 310-10-35	83,379		97,373
Ending Balance Evaluated for Impairment Under ASC 450-20	5,242,312		5,256,329
Total	5,325,691		5,353,702
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	3,803,011		3,825,847
Agricultural and agricultural real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	143		816
Ending Balance Under ASC 450-20	3,689		3,394
Total	3,832	3,983	4,210	4,100	4,028	3,887
Ending Balance Evaluated for Impairment Under ASC 310-10-35	11,781		16,730
Ending Balance Evaluated for Impairment Under ASC 450-20	483,462		472,588
Total	495,243		489,318
Residential real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	416		497
Ending Balance Under ASC 450-20	1,847		1,766
Total	2,263	2,183	2,263	2,065	1,851	1,934
Ending Balance Evaluated for Impairment Under ASC 310-10-35	28,757		25,726
Ending Balance Evaluated for Impairment Under ASC 450-20	567,628		592,198
Total	596,385		617,924
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	1,207		1,451
Ending Balance Under ASC 450-20	7,720		7,316
Total	8,927	8,856	8,767	7,098	6,990	7,237
Ending Balance Evaluated for Impairment Under ASC 310-10-35	5,904		5,988
Ending Balance Evaluated for Impairment Under ASC 450-20	425,148		414,625
Total	431,052		420,613
Commercial | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	2,164		1,318
Ending Balance Under ASC 450-20	15,004		13,447
Total	17,168	16,180	14,765	15,525	16,374	16,095
Ending Balance Evaluated for Impairment Under ASC 310-10-35	7,298		3,712
Ending Balance Evaluated for Impairment Under ASC 450-20	1,337,025		1,283,553
Total	1,344,323		1,287,265
Commercial real estate | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	3,018		2,671
Ending Balance Under ASC 450-20	18,843		21,648
Total	21,861	$ 23,797	24,319	$ 22,968	$ 20,495	$ 19,532
Ending Balance Evaluated for Impairment Under ASC 310-10-35	29,639		45,217
Ending Balance Evaluated for Impairment Under ASC 450-20	2,429,049		2,493,365
Total	$ 2,458,688		$ 2,538,582