Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				$ 55,686				$ 54,324	$ 55,686	$ 54,324	$ 48,685
Provision for loan losses	$ 9,681	$ 5,238	$ 4,831	4,263	$ 5,328	$ 5,705	$ 889	3,641	24,013	15,563	11,694
Recoveries on loans previously charged-off									3,549	3,670	5,339
Loans charged-off									(21,285)	(17,871)	(11,394)
Balance at end of year	61,963				55,686				61,963	55,686	54,324
Commercial | Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				18,098				14,765	18,098	14,765
Provision for loan losses									13,345	7,162
Recoveries on loans previously charged-off									978	811
Loans charged-off									(7,916)	(4,640)
Balance at end of year	24,505				18,098				24,505	18,098	14,765
Commercial | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				21,950				24,319	21,950	24,319
Provision for loan losses									4,518	(849)
Recoveries on loans previously charged-off									1,047	1,192
Loans charged-off									(1,977)	(2,712)
Balance at end of year	25,538				21,950				25,538	21,950	24,319
Agricultural and agricultural real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				4,258				4,210	4,258	4,210
Provision for loan losses									2,119	2,946
Recoveries on loans previously charged-off									13	18
Loans charged-off									(1,437)	(2,916)
Balance at end of year	4,953				4,258				4,953	4,258	4,210
Residential real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				2,224				2,263	2,224	2,263
Provision for loan losses									(163)	403
Recoveries on loans previously charged-off									96	358
Loans charged-off									(372)	(800)
Balance at end of year	1,785				2,224				1,785	2,224	2,263
Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				$ 9,156				$ 8,767	9,156	8,767
Provision for loan losses									4,194	5,901
Recoveries on loans previously charged-off									1,415	1,291
Loans charged-off									(9,583)	(6,803)
Balance at end of year	$ 5,182				$ 9,156				$ 5,182	$ 9,156	$ 8,767