Loans (Loans not covered by share agreements (credit quality indicator)) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	$ 5,441,546	$ 5,003,866
Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	5,044,115	4,541,863
Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	397,431	462,003
Commercial
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	3,900,612	3,605,574
Commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	3,608,907	3,213,750
Commercial | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	291,705	391,824
Commercial | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	1,295,316	1,279,214
Commercial | Commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	1,196,389	1,106,276
Commercial | Commercial | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	98,927	172,938
Commercial | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	2,605,296	2,326,360
Commercial | Commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	2,412,518	2,107,474
Commercial | Commercial real estate | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	192,778	218,886
Agricultural and agricultural real estate
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	489,387	471,870
Agricultural and agricultural real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	422,810	435,745
Agricultural and agricultural real estate | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	66,577	36,125
Residential real estate
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	625,965	539,555
Residential real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	596,479	515,195
Residential real estate | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	29,486	24,360
Consumer
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	425,582	386,867
Consumer | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	415,919	377,173
Consumer | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	$ 9,663	$ 9,694