Loans (Loans by credit quality indicator) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	$ 7,368,123	$ 6,393,226
Loans delinquent 30 to 89 days, percentage	0.62%	0.27%
Loans secured by residential real estate property in process of foreclosure	$ 3,100
Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	6,905,159	$ 6,009,382
Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	$ 462,964	$ 383,844
Percent of nonaccrual loans	16.00%	16.00%
Special Mention
Financing Receivable, Recorded Investment [Line Items]
Percent of loans	53.00%	52.00%
Substandard
Financing Receivable, Recorded Investment [Line Items]
Percent of loans	47.00%	48.00%
Commercial
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	$ 5,610,953	$ 4,809,875
Commercial \| Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	5,293,214	4,538,284
Commercial \| Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	317,739	271,591
Commercial \| Commercial
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	1,962,222	1,646,606
Commercial \| Commercial \| Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	1,825,692	1,552,783
Commercial \| Commercial \| Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	136,530	93,823
Commercial \| Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	3,648,731	3,163,269
Commercial \| Commercial real estate \| Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	3,467,522	2,985,501
Commercial \| Commercial real estate \| Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	181,209	177,768
Agricultural and agricultural real estate
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	574,048	511,588
Agricultural and agricultural real estate \| Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	476,261	451,539
Agricultural and agricultural real estate \| Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	97,787	60,049
Residential real estate
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	676,941	624,279
Residential real estate \| Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	644,592	586,623
Residential real estate \| Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	32,349	37,656
Consumer
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	506,181	447,484
Consumer \| Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	491,092	432,936
Consumer \| Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	$ 15,089	$ 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

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- Definition

Loans And Leases Receivable, Gross, Percentage

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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

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-Publisher FASB
-Name Accounting Standards Codification
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+ References

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