Loans (Allowance for credit losses on financing receivables) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	$ 7,542		$ 2,768
Ending Balance Under ASC 450-20	47,111		45,917
Total	54,653	$ 51,756	48,685	$ 47,105	$ 45,614	$ 41,449
Ending Balance Evaluated for Impairment Under ASC 310-10-35	110,357		80,221
Ending Balance Evaluated for Impairment Under ASC 450-20	5,331,189		4,923,645
Total	5,441,546		5,003,866
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	3,900,612		3,605,574
Commercial | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	2,077		471
Ending Balance Under ASC 450-20	14,814		15,624
Total	16,891	15,525	16,095	13,910	13,064	11,909
Ending Balance Evaluated for Impairment Under ASC 310-10-35	8,002		6,919
Ending Balance Evaluated for Impairment Under ASC 450-20	1,287,314		1,272,295
Total	1,295,316		1,279,214
Commercial | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	2,661		698
Ending Balance Under ASC 450-20	21,020		18,834
Total	23,681	22,968	19,532	18,086	17,608	15,898
Ending Balance Evaluated for Impairment Under ASC 310-10-35	56,894		45,442
Ending Balance Evaluated for Impairment Under ASC 450-20	2,548,402		2,280,918
Total	2,605,296		2,326,360
Agricultural and agricultural real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	967		0
Ending Balance Under ASC 450-20	4,039		3,887
Total	5,006	4,100	3,887	3,939	3,676	3,295
Ending Balance Evaluated for Impairment Under ASC 310-10-35	17,155		4,612
Ending Balance Evaluated for Impairment Under ASC 450-20	472,232		467,258
Total	489,387		471,870
Residential real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	473		393
Ending Balance Under ASC 450-20	1,509		1,541
Total	1,982	2,065	1,934	3,846	4,099	3,741
Ending Balance Evaluated for Impairment Under ASC 310-10-35	22,448		17,790
Ending Balance Evaluated for Impairment Under ASC 450-20	603,517		521,765
Total	625,965		539,555
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	1,364		1,206
Ending Balance Under ASC 450-20	5,729		6,031
Total	7,093	$ 7,098	7,237	$ 7,324	$ 7,167	$ 6,606
Ending Balance Evaluated for Impairment Under ASC 310-10-35	5,858		5,458
Ending Balance Evaluated for Impairment Under ASC 450-20	419,724		381,409
Total	$ 425,582		$ 386,867