Loans (Past due financing receivables) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	$ 14,222	$ 18,137
Accruing Loans and Leases, Current	6,668,789	6,312,508
Nonaccrual	64,806	62,581
Total	6,747,817	6,393,226
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	9,965	13,375
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	4,235	3,932
90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	22	830
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	5,932	8,700
Accruing Loans and Leases, Current	5,098,608	4,777,349
Nonaccrual	25,237	23,826
Total	5,129,777	4,809,875
Commercial | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	3,309	6,015
Commercial | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	2,623	2,585
Commercial | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	0	100
Commercial | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	4,789	1,605
Accruing Loans and Leases, Current	1,793,565	1,637,773
Nonaccrual	8,329	7,228
Total	1,806,683	1,646,606
Commercial | Commercial | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	2,906	1,246
Commercial | Commercial | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	1,883	259
Commercial | Commercial | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	0	100
Commercial | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	1,143	7,095
Accruing Loans and Leases, Current	3,305,043	3,139,576
Nonaccrual	16,908	16,598
Total	3,323,094	3,163,269
Commercial | Commercial real estate | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	403	4,769
Commercial | Commercial real estate | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	740	2,326
Commercial | Commercial real estate | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	0	0
Agricultural and agricultural real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	1,238	738
Accruing Loans and Leases, Current	500,320	497,546
Nonaccrual	16,828	13,304
Total	518,386	511,588
Agricultural and agricultural real estate | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	1,147	604
Agricultural and agricultural real estate | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	69	134
Agricultural and agricultural real estate | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	22	0
Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	2,957	2,292
Accruing Loans and Leases, Current	602,927	601,120
Nonaccrual	18,841	20,867
Total	624,725	624,279
Residential real estate | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	2,891	2,022
Residential real estate | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	66	270
Residential real estate | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	0	0
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	4,095	6,407
Accruing Loans and Leases, Current	466,934	436,493
Nonaccrual	3,900	4,584
Total	474,929	447,484
Consumer | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	2,618	4,734
Consumer | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	1,477	943
Consumer | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	$ 0	$ 730