Loans (Credit quality indicator) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	$ 6,375,319	$ 5,353,702
Loans delinquent 30 to 89 days, percentage	0.33%	0.37%
Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	$ 5,997,115	$ 4,985,600
Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	$ 378,204	$ 368,102
Percent of nonaccrual loans	17.00%	17.00%
Special Mention
Financing Receivable, Recorded Investment [Line Items]
Percent of loans	48.00%	47.00%
Substandard
Financing Receivable, Recorded Investment [Line Items]
Percent of loans	52.00%	53.00%
Commercial
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	$ 4,777,856	$ 3,825,847
Commercial \| Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	4,515,743	3,567,189
Commercial \| Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	262,113	258,658
Commercial \| Commercial
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	1,613,903	1,287,265
Commercial \| Commercial \| Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	1,523,080	1,187,557
Commercial \| Commercial \| Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	90,823	99,708
Commercial \| Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	3,163,953	2,538,582
Commercial \| Commercial real estate \| Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	2,992,663	2,379,632
Commercial \| Commercial real estate \| Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	171,290	158,950
Agricultural and agricultural real estate
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	511,764	489,318
Agricultural and agricultural real estate \| Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	445,554	424,311
Agricultural and agricultural real estate \| Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	66,210	65,007
Residential real estate
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	635,611	617,924
Residential real estate \| Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	597,987	584,626
Residential real estate \| Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	37,624	33,298
Consumer
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	450,088	420,613
Consumer \| Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	437,831	409,474
Consumer \| Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	$ 12,257	$ 11,139

Financing Receivable, Recorded Investment, 30 to 89 Days Past Due, Percentage

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- Definition

Financing Receivables, Recorded Investments, Nonaccrual Status, Percentage

+ References

No definition available.

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- Definition

Loans And Leases Receivable, Gross, Percentage

+ References

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- Definition

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+ References

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- Definition

Amount representing an agreement for an unconditional promise by the maker to pay the entity (holder) a definite sum of money at a future date. Such amount may include accrued interest receivable in accordance with the terms of the note. The note also may contain provisions and related items including a discount or premium, payable on demand, secured, or unsecured, interest bearing or noninterest bearing, among a myriad of other features and characteristics. Excludes amounts related to receivables held-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.10(3))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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