Loans (Carrying amount of loans acquired) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	$ 2,109,159	$ 1,470,025
Commercial | Commercial
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	401,837	188,327
Commercial | Commercial real estate
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	1,351,160	1,055,041
Agricultural and agricultural real estate
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	31,010	1,242
Residential real estate
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	220,664	174,123
Consumer
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	104,488	51,292
Impaired Purchased Loans
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	11,736	3,738
Impaired Purchased Loans | Commercial | Commercial
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	10,150	952
Impaired Purchased Loans | Commercial | Commercial real estate
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	1,376	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	210	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	2,097,423	1,466,287
Non Impaired Purchased Loans | Commercial | Commercial
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	391,687	187,375
Non Impaired Purchased Loans | Commercial | Commercial real estate
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	1,349,784	1,052,469
Non Impaired Purchased Loans | Agricultural and agricultural real estate
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	31,010	1,242
Non Impaired Purchased Loans | Residential real estate
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	220,454	173,909
Non Impaired Purchased Loans | Consumer
Loans covered by loss share agreement (carrying amount) [Line Items]
Purchased loans	$ 104,488	$ 51,292