Loans (Carrying amount of loans acquired) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	$ 1,838,230	$ 1,470,025
Commercial | Commercial
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	309,339	188,327
Commercial | Commercial real estate
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	1,202,839	1,055,041
Agricultural and agricultural real estate
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	32,357	1,242
Residential real estate
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	204,634	174,123
Consumer
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	89,061	51,292
Impaired Purchased Loans
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	9,600	3,738
Impaired Purchased Loans | Commercial | Commercial
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	9,078	952
Impaired Purchased Loans | Commercial | Commercial real estate
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	302	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	220	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	1,828,630	1,466,287
Non Impaired Purchased Loans | Commercial | Commercial
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	300,261	187,375
Non Impaired Purchased Loans | Commercial | Commercial real estate
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	1,202,537	1,052,469
Non Impaired Purchased Loans | Agricultural and agricultural real estate
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	32,357	1,242
Non Impaired Purchased Loans | Residential real estate
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	204,414	173,909
Non Impaired Purchased Loans | Consumer
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	$ 89,061	$ 51,292