Loans (Past due financing receivables) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	$ 45,593	$ 18,137
Accruing Loans and Leases, Current	7,249,470	6,312,508
Nonaccrual	73,060	62,581
Total	7,368,123	6,393,226
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	30,733	13,375
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	14,706	3,932
90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	154	830
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	34,729	8,700
Accruing Loans and Leases, Current	5,540,910	4,777,349
Nonaccrual	35,314	23,826
Total	5,610,953	4,809,875
Commercial | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	22,053	6,015
Commercial | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	12,557	2,585
Commercial | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	119	100
Commercial | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	14,140	1,605
Accruing Loans and Leases, Current	1,924,173	1,637,773
Nonaccrual	23,909	7,228
Total	1,962,222	1,646,606
Commercial | Commercial | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	10,558	1,246
Commercial | Commercial | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	3,521	259
Commercial | Commercial | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	61	100
Commercial | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	20,589	7,095
Accruing Loans and Leases, Current	3,616,737	3,139,576
Nonaccrual	11,405	16,598
Total	3,648,731	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	11,495	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	9,036	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	58	0
Agricultural and agricultural real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	2,475	738
Accruing Loans and Leases, Current	554,055	497,546
Nonaccrual	17,518	13,304
Total	574,048	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	1,294	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	1,146	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	35	0
Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	3,085	2,292
Accruing Loans and Leases, Current	659,662	601,120
Nonaccrual	14,194	20,867
Total	676,941	624,279
Residential real estate | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	2,893	2,022
Residential real estate | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	192	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	5,304	6,407
Accruing Loans and Leases, Current	494,843	436,493
Nonaccrual	6,034	4,584
Total	506,181	447,484
Consumer | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	4,493	4,734
Consumer | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	811	943
Consumer | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	$ 0	$ 730