Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				$ 54,324				$ 48,685	$ 54,324	$ 48,685	$ 41,449
Provision for loan losses	$ 5,328	$ 5,705	$ 889	3,641	$ 2,181	$ 5,328	$ 2,118	2,067	15,563	11,694	12,697
Recoveries on loans previously charged-off									3,670	5,339	3,553
Loans charged-off									(17,871)	(11,394)	(9,014)
Balance at end of year	55,686				54,324				55,686	54,324	48,685
Commercial | Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				14,765				16,095	14,765	16,095
Provision for loan losses									7,162	(912)
Recoveries on loans previously charged-off									811	930
Loans charged-off									(4,640)	(1,348)
Balance at end of year	18,098				14,765				18,098	14,765	16,095
Commercial | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				24,319				19,532	24,319	19,532
Provision for loan losses									(849)	4,328
Recoveries on loans previously charged-off									1,192	3,327
Loans charged-off									(2,712)	(2,868)
Balance at end of year	21,950				24,319				21,950	24,319	19,532
Agricultural and agricultural real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				4,210				3,887	4,210	3,887
Provision for loan losses									2,946	527
Recoveries on loans previously charged-off									18	10
Loans charged-off									(2,916)	(214)
Balance at end of year	4,258				4,210				4,258	4,210	3,887
Residential real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				2,263				1,934	2,263	1,934
Provision for loan losses									403	646
Recoveries on loans previously charged-off									358	29
Loans charged-off									(800)	(346)
Balance at end of year	2,224				2,263				2,224	2,263	1,934
Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				$ 8,767				$ 7,237	8,767	7,237
Provision for loan losses									5,901	7,105
Recoveries on loans previously charged-off									1,291	1,043
Loans charged-off									(6,803)	(6,618)
Balance at end of year	$ 9,156				$ 8,767				$ 9,156	$ 8,767	$ 7,237