Loans (Allowance for credit losses on financing receivables) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	$ 7,043		$ 6,753
Ending Balance Under ASC 450-20	47,842		47,571
Total	54,885	$ 54,051	54,324	$ 54,653	$ 51,756	$ 48,685
Ending Balance Evaluated for Impairment Under ASC 310-10-35	84,049		97,373
Ending Balance Evaluated for Impairment Under ASC 450-20	6,291,270		5,256,329
Total	6,375,319		5,353,702
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	4,777,856		3,825,847
Commercial | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	2,166		1,318
Ending Balance Under ASC 450-20	14,804		13,447
Total	16,970	17,168	14,765	16,891	15,525	16,095
Ending Balance Evaluated for Impairment Under ASC 310-10-35	6,957		3,712
Ending Balance Evaluated for Impairment Under ASC 450-20	1,606,946		1,283,553
Total	1,613,903		1,287,265
Commercial | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	864		2,671
Ending Balance Under ASC 450-20	19,676		21,648
Total	20,540	21,861	24,319	23,681	22,968	19,532
Ending Balance Evaluated for Impairment Under ASC 310-10-35	27,943		45,217
Ending Balance Evaluated for Impairment Under ASC 450-20	3,136,010		2,493,365
Total	3,163,953		2,538,582
Agricultural and agricultural real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	2,353		816
Ending Balance Under ASC 450-20	3,774		3,394
Total	6,127	3,832	4,210	5,006	4,100	3,887
Ending Balance Evaluated for Impairment Under ASC 310-10-35	12,792		16,730
Ending Balance Evaluated for Impairment Under ASC 450-20	498,972		472,588
Total	511,764		489,318
Residential real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	393		497
Ending Balance Under ASC 450-20	1,873		1,766
Total	2,266	2,263	2,263	1,982	2,065	1,934
Ending Balance Evaluated for Impairment Under ASC 310-10-35	29,833		25,726
Ending Balance Evaluated for Impairment Under ASC 450-20	605,778		592,198
Total	635,611		617,924
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	1,267		1,451
Ending Balance Under ASC 450-20	7,715		7,316
Total	8,982	$ 8,927	8,767	$ 7,093	$ 7,098	$ 7,237
Ending Balance Evaluated for Impairment Under ASC 310-10-35	6,524		5,988
Ending Balance Evaluated for Impairment Under ASC 450-20	443,564		414,625
Total	$ 450,088		$ 420,613