Loans (Loans by credit quality indicator) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	$ 6,747,817	$ 6,393,226
Loans delinquent 30 to 89 days, percentage	0.21%	0.27%
Loans secured by residential real estate property in process of foreclosure	$ 2,800
Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	6,314,116	$ 6,009,382
Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	$ 433,701	$ 383,844
Percent of nonaccrual loans	15.00%	16.00%
Special Mention
Financing Receivable, Recorded Investment [Line Items]
Percent of loans	55.00%	52.00%
Substandard
Financing Receivable, Recorded Investment [Line Items]
Percent of loans	45.00%	48.00%
Commercial
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	$ 5,129,777	$ 4,809,875
Commercial \| Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	4,823,960	4,538,284
Commercial \| Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	305,817	271,591
Commercial \| Commercial
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	1,806,683	1,646,606
Commercial \| Commercial \| Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	1,677,338	1,552,783
Commercial \| Commercial \| Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	129,345	93,823
Commercial \| Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	3,323,094	3,163,269
Commercial \| Commercial real estate \| Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	3,146,622	2,985,501
Commercial \| Commercial real estate \| Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	176,472	177,768
Agricultural and agricultural real estate
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	518,386	511,588
Agricultural and agricultural real estate \| Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	442,484	451,539
Agricultural and agricultural real estate \| Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	75,902	60,049
Residential real estate
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	624,725	624,279
Residential real estate \| Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	585,886	586,623
Residential real estate \| Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	38,839	37,656
Consumer
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	474,929	447,484
Consumer \| Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	461,786	432,936
Consumer \| Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	$ 13,143	$ 14,548

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

Recorded investment of consumer mortgage loan receivables secured by residential real estate properties for which formal foreclosure proceedings are in process.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 35
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL49126937-111524

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+ 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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