Loans (Carrying amount of loans acquired) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	$ 1,606,305	$ 1,470,025
Commercial | Commercial
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	265,249	188,327
Commercial | Commercial real estate
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	1,081,198	1,055,041
Agricultural and agricultural real estate
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	26	1,242
Residential real estate
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	181,219	174,123
Consumer
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	78,613	51,292
Impaired Purchased Loans
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	5,815	3,738
Impaired Purchased Loans | Commercial | Commercial
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	3,142	952
Impaired Purchased Loans | Commercial | Commercial real estate
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	2,474	2,572
Impaired Purchased Loans | Agricultural and agricultural real estate
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	0	0
Impaired Purchased Loans | Residential real estate
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	199	214
Impaired Purchased Loans | Consumer
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	0	0
Non Impaired Purchased Loans
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	1,600,490	1,466,287
Non Impaired Purchased Loans | Commercial | Commercial
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	262,107	187,375
Non Impaired Purchased Loans | Commercial | Commercial real estate
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	1,078,724	1,052,469
Non Impaired Purchased Loans | Agricultural and agricultural real estate
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	26	1,242
Non Impaired Purchased Loans | Residential real estate
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	181,020	173,909
Non Impaired Purchased Loans | Consumer
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	$ 78,613	$ 51,292