Loans (Past due financing receivables) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	$ 23,070	$ 19,945
Accruing Loans and Leases, Current	6,288,793	5,269,458
Nonaccrual	63,456	64,299
Total	6,375,319	5,353,702
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	17,080	10,653
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	3,642	9,206
90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	2,348	86
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	9,544	6,238
Accruing Loans and Leases, Current	4,744,069	3,794,310
Nonaccrual	24,243	25,299
Total	4,777,856	3,825,847
Commercial | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	8,731	2,013
Commercial | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	598	4,148
Commercial | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	215	77
Commercial | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	2,939	1,423
Accruing Loans and Leases, Current	1,603,397	1,281,241
Nonaccrual	7,567	4,601
Total	1,613,903	1,287,265
Commercial | Commercial | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	2,591	1,127
Commercial | Commercial | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	133	219
Commercial | Commercial | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	215	77
Commercial | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	6,605	4,815
Accruing Loans and Leases, Current	3,140,672	2,513,069
Nonaccrual	16,676	20,698
Total	3,163,953	2,538,582
Commercial | Commercial real estate | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	6,140	886
Commercial | Commercial real estate | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	465	3,929
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	2,379	3,390
Accruing Loans and Leases, Current	496,593	472,597
Nonaccrual	12,792	13,331
Total	511,764	489,318
Agricultural and agricultural real estate | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	315	199
Agricultural and agricultural real estate | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	782	3,191
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	1,282	0
Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	5,482	5,832
Accruing Loans and Leases, Current	607,165	590,626
Nonaccrual	22,964	21,466
Total	635,611	617,924
Residential real estate | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	5,033	4,986
Residential real estate | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	449	846
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	5,665	4,485
Accruing Loans and Leases, Current	440,966	411,925
Nonaccrual	3,457	4,203
Total	450,088	420,613
Consumer | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	3,001	3,455
Consumer | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	1,813	1,021
Consumer | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans and Leases, Past Due	$ 851	$ 9