Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				$ 48,685				$ 41,449	$ 48,685	$ 41,449	$ 41,685
Provision for loan losses	$ 2,181	$ 5,328	$ 2,118	2,067	$ 2,171	$ 3,181	$ 5,674	1,671	11,694	12,697	14,501
Recoveries on loans previously charged-off									5,339	3,553	3,990
Loans charged-off									(11,394)	(9,014)	(18,727)
Balance at end of year	54,324				48,685				54,324	48,685	41,449
Commercial | Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				16,095				11,909	16,095	11,909
Provision for loan losses									(912)	4,906
Recoveries on loans previously charged-off									930	1,167
Loans charged-off									(1,348)	(1,887)
Balance at end of year	14,765				16,095				14,765	16,095	11,909
Commercial | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				19,532				15,898	19,532	15,898
Provision for loan losses									4,328	3,802
Recoveries on loans previously charged-off									3,327	1,200
Loans charged-off									(2,868)	(1,368)
Balance at end of year	24,319				19,532				24,319	19,532	15,898
Agricultural and agricultural real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				3,887				3,295	3,887	3,295
Provision for loan losses									527	1,111
Recoveries on loans previously charged-off									10	32
Loans charged-off									(214)	(551)
Balance at end of year	4,210				3,887				4,210	3,887	3,295
Residential real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				1,934				3,741	1,934	3,741
Provision for loan losses									646	(1,749)
Recoveries on loans previously charged-off									29	183
Loans charged-off									(346)	(241)
Balance at end of year	2,263				1,934				2,263	1,934	3,741
Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year				$ 7,237				$ 6,606	7,237	6,606
Provision for loan losses									7,105	4,627
Recoveries on loans previously charged-off									1,043	971
Loans charged-off									(6,618)	(4,967)
Balance at end of year	$ 8,767				$ 7,237				$ 8,767	$ 7,237	$ 6,606