Loans - (Credit Exposure for Loans as Determined by Company's Internal Risk Rating System) (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Loans [Line Items]
Total Loans	$ 2,587,673	$ 2,162,409
Commercial Loan [Member]
Loans [Line Items]
Total Loans	1,052,776	795,396
Agriculture [Member]
Loans [Line Items]
Total Loans	162,310	137,531
Residential Real Estate Segment [Member]
Loans [Line Items]
Total Loans	684,002	632,700
Consumer [Member]
Loans [Line Items]
Total Loans	33,366	35,188
Commercial Real Estate [Member]
Loans [Line Items]
Total Loans	655,219	561,594
Non ASC 310-30 [Member]
Loans [Line Items]
Total Loans	2,384,843	1,882,764
Non ASC 310-30 [Member] | Commercial Loan [Member]
Loans [Line Items]
Total Loans	1,039,769	772,440
Non ASC 310-30 [Member] | Agriculture [Member]
Loans [Line Items]
Total Loans	145,558	118,468
Non ASC 310-30 [Member] | Residential Real Estate Segment [Member]
Loans [Line Items]
Total Loans	662,550	591,939
Non ASC 310-30 [Member] | Consumer [Member]
Loans [Line Items]
Total Loans	30,635	30,653
Non ASC 310-30 [Member] | Commercial Real Estate [Member]
Loans [Line Items]
Total Loans	506,331	369,264
ASC 310-30 [Member]
Loans [Line Items]
Total Loans	202,830	279,645
ASC 310-30 [Member] | Commercial Loan [Member]
Loans [Line Items]
Total Loans	13,007	22,956
ASC 310-30 [Member] | Agriculture [Member]
Loans [Line Items]
Total Loans	16,752	19,063
ASC 310-30 [Member] | Residential Real Estate Segment [Member]
Loans [Line Items]
Total Loans	21,452	40,761
ASC 310-30 [Member] | Consumer [Member]
Loans [Line Items]
Total Loans	2,731	4,535
ASC 310-30 [Member] | Commercial Real Estate [Member]
Loans [Line Items]
Total Loans	148,888	192,330
Total Loans [Member]
Loans [Line Items]
Total Loans	2,587,673	2,162,409
Total Loans [Member] | Non ASC 310-30 [Member]
Loans [Line Items]
Total Loans	2,384,843	1,882,764
Total Loans [Member] | Non ASC 310-30 [Member] | Commercial Loan [Member]
Loans [Line Items]
Total Loans	1,039,769	772,440
Total Loans [Member] | Non ASC 310-30 [Member] | Agriculture [Member]
Loans [Line Items]
Total Loans	145,558	118,468
Total Loans [Member] | Non ASC 310-30 [Member] | Total Commercial Real Estate [Member]
Loans [Line Items]
Total Loans	506,331	369,264
Total Loans [Member] | Non ASC 310-30 [Member] | Commercial Construction [Member]
Loans [Line Items]
Total Loans	30,143	11,748
Total Loans [Member] | Non ASC 310-30 [Member] | Commercial Acquisition/Development [Member]
Loans [Line Items]
Total Loans	5,575	4,573
Total Loans [Member] | Non ASC 310-30 [Member] | Multifamily [Member]
Loans [Line Items]
Total Loans	9,873	10,856
Total Loans [Member] | Non ASC 310-30 [Member] | Owner-Occupied [Member]
Loans [Line Items]
Total Loans	184,619	120,225
Total Loans [Member] | Non ASC 310-30 [Member] | Non Owner-Occupied [Member]
Loans [Line Items]
Total Loans	276,121	221,862
Total Loans [Member] | Non ASC 310-30 [Member] | Residential Real Estate Segment [Member]
Loans [Line Items]
Total Loans	662,550	591,939
Total Loans [Member] | Non ASC 310-30 [Member] | Senior lien
Loans [Line Items]
Total Loans	614,493	539,535
Total Loans [Member] | Non ASC 310-30 [Member] | Junior lien
Loans [Line Items]
Total Loans	48,057	52,404
Total Loans [Member] | Non ASC 310-30 [Member] | Consumer [Member]
Loans [Line Items]
Total Loans	30,635	30,653
Total Loans [Member] | ASC 310-30 [Member]
Loans [Line Items]
Total Loans	202,830	279,645
Total Loans [Member] | ASC 310-30 [Member] | Commercial Loan [Member]
Loans [Line Items]
Total Loans	13,007	22,956
Total Loans [Member] | ASC 310-30 [Member] | Agriculture [Member]
Loans [Line Items]
Total Loans	16,752	19,063
Total Loans [Member] | ASC 310-30 [Member] | Residential Real Estate Segment [Member]
Loans [Line Items]
Total Loans	21,452	40,761
Total Loans [Member] | ASC 310-30 [Member] | Consumer [Member]
Loans [Line Items]
Total Loans	2,731	4,535
Total Loans [Member] | ASC 310-30 [Member] | Commercial Real Estate [Member]
Loans [Line Items]
Total Loans	148,888	192,330
Total Loans [Member] | Pass [Member]
Loans [Line Items]
Total Loans	2,356,761	1,958,078
Total Loans [Member] | Pass [Member] | Non ASC 310-30 [Member]
Loans [Line Items]
Total Loans	2,252,304	1,814,873
Total Loans [Member] | Pass [Member] | Non ASC 310-30 [Member] | Commercial Loan [Member]
Loans [Line Items]
Total Loans	957,992	742,944
Total Loans [Member] | Pass [Member] | Non ASC 310-30 [Member] | Agriculture [Member]
Loans [Line Items]
Total Loans	132,450	114,642
Total Loans [Member] | Pass [Member] | Non ASC 310-30 [Member] | Total Commercial Real Estate [Member]
Loans [Line Items]
Total Loans	475,746	342,172
Total Loans [Member] | Pass [Member] | Non ASC 310-30 [Member] | Commercial Construction [Member]
Loans [Line Items]
Total Loans	24,686	11,748
Total Loans [Member] | Pass [Member] | Non ASC 310-30 [Member] | Commercial Acquisition/Development [Member]
Loans [Line Items]
Total Loans	5,066	4,573
Total Loans [Member] | Pass [Member] | Non ASC 310-30 [Member] | Multifamily [Member]
Loans [Line Items]
Total Loans	9,851	10,856
Total Loans [Member] | Pass [Member] | Non ASC 310-30 [Member] | Owner-Occupied [Member]
Loans [Line Items]
Total Loans	174,108	115,178
Total Loans [Member] | Pass [Member] | Non ASC 310-30 [Member] | Non Owner-Occupied [Member]
Loans [Line Items]
Total Loans	262,035	199,817
Total Loans [Member] | Pass [Member] | Non ASC 310-30 [Member] | Residential Real Estate Segment [Member]
Loans [Line Items]
Total Loans	655,633	584,689
Total Loans [Member] | Pass [Member] | Non ASC 310-30 [Member] | Senior lien
Loans [Line Items]
Total Loans	609,196	533,630
Total Loans [Member] | Pass [Member] | Non ASC 310-30 [Member] | Junior lien
Loans [Line Items]
Total Loans	46,437	51,059
Total Loans [Member] | Pass [Member] | Non ASC 310-30 [Member] | Consumer [Member]
Loans [Line Items]
Total Loans	30,483	30,426
Total Loans [Member] | Pass [Member] | ASC 310-30 [Member]
Loans [Line Items]
Total Loans	104,457	143,205
Total Loans [Member] | Pass [Member] | ASC 310-30 [Member] | Commercial Loan [Member]
Loans [Line Items]
Total Loans	8,236	11,038
Total Loans [Member] | Pass [Member] | ASC 310-30 [Member] | Agriculture [Member]
Loans [Line Items]
Total Loans	9,726	16,854
Total Loans [Member] | Pass [Member] | ASC 310-30 [Member] | Residential Real Estate Segment [Member]
Loans [Line Items]
Total Loans	16,960	29,069
Total Loans [Member] | Pass [Member] | ASC 310-30 [Member] | Consumer [Member]
Loans [Line Items]
Total Loans	2,296	3,641
Total Loans [Member] | Pass [Member] | ASC 310-30 [Member] | Commercial Real Estate [Member]
Loans [Line Items]
Total Loans	67,239	82,603
Total Loans [Member] | Special Mention [Member]
Loans [Line Items]
Total Loans	69,888	33,629
Total Loans [Member] | Special Mention [Member] | Non ASC 310-30 [Member]
Loans [Line Items]
Total Loans	67,051	28,039
Total Loans [Member] | Special Mention [Member] | Non ASC 310-30 [Member] | Commercial Loan [Member]
Loans [Line Items]
Total Loans	44,866	10,166
Total Loans [Member] | Special Mention [Member] | Non ASC 310-30 [Member] | Agriculture [Member]
Loans [Line Items]
Total Loans	2,440	85
Total Loans [Member] | Special Mention [Member] | Non ASC 310-30 [Member] | Total Commercial Real Estate [Member]
Loans [Line Items]
Total Loans	19,077	17,765
Total Loans [Member] | Special Mention [Member] | Non ASC 310-30 [Member] | Commercial Construction [Member]
Loans [Line Items]
Total Loans	4,882
Total Loans [Member] | Special Mention [Member] | Non ASC 310-30 [Member] | Commercial Acquisition/Development [Member]
Loans [Line Items]
Total Loans	509
Total Loans [Member] | Special Mention [Member] | Non ASC 310-30 [Member] | Owner-Occupied [Member]
Loans [Line Items]
Total Loans	5,595	158
Total Loans [Member] | Special Mention [Member] | Non ASC 310-30 [Member] | Non Owner-Occupied [Member]
Loans [Line Items]
Total Loans	8,091	17,607
Total Loans [Member] | Special Mention [Member] | Non ASC 310-30 [Member] | Residential Real Estate Segment [Member]
Loans [Line Items]
Total Loans	601	23
Total Loans [Member] | Special Mention [Member] | Non ASC 310-30 [Member] | Senior lien
Loans [Line Items]
Total Loans	349	23
Total Loans [Member] | Special Mention [Member] | Non ASC 310-30 [Member] | Junior lien
Loans [Line Items]
Total Loans	252
Total Loans [Member] | Special Mention [Member] | Non ASC 310-30 [Member] | Consumer [Member]
Loans [Line Items]
Total Loans	67
Total Loans [Member] | Special Mention [Member] | ASC 310-30 [Member]
Loans [Line Items]
Total Loans	2,837	5,590
Total Loans [Member] | Special Mention [Member] | ASC 310-30 [Member] | Commercial Loan [Member]
Loans [Line Items]
Total Loans	25	282
Total Loans [Member] | Special Mention [Member] | ASC 310-30 [Member] | Agriculture [Member]
Loans [Line Items]
Total Loans	27	30
Total Loans [Member] | Special Mention [Member] | ASC 310-30 [Member] | Residential Real Estate Segment [Member]
Loans [Line Items]
Total Loans	1,604	1,403
Total Loans [Member] | Special Mention [Member] | ASC 310-30 [Member] | Consumer [Member]
Loans [Line Items]
Total Loans	94	105
Total Loans [Member] | Special Mention [Member] | ASC 310-30 [Member] | Commercial Real Estate [Member]
Loans [Line Items]
Total Loans	1,087	3,770
Total Loans [Member] | Substandard [Member]
Loans [Line Items]
Total Loans	152,911	165,941
Total Loans [Member] | Substandard [Member] | Non ASC 310-30 [Member]
Loans [Line Items]
Total Loans	61,144	39,626
Total Loans [Member] | Substandard [Member] | Non ASC 310-30 [Member] | Commercial Loan [Member]
Loans [Line Items]
Total Loans	32,867	19,250
Total Loans [Member] | Substandard [Member] | Non ASC 310-30 [Member] | Agriculture [Member]
Loans [Line Items]
Total Loans	10,668	3,741
Total Loans [Member] | Substandard [Member] | Non ASC 310-30 [Member] | Total Commercial Real Estate [Member]
Loans [Line Items]
Total Loans	11,235	9,319
Total Loans [Member] | Substandard [Member] | Non ASC 310-30 [Member] | Commercial Construction [Member]
Loans [Line Items]
Total Loans	575
Total Loans [Member] | Substandard [Member] | Non ASC 310-30 [Member] | Multifamily [Member]
Loans [Line Items]
Total Loans	22
Total Loans [Member] | Substandard [Member] | Non ASC 310-30 [Member] | Owner-Occupied [Member]
Loans [Line Items]
Total Loans	4,916	4,889
Total Loans [Member] | Substandard [Member] | Non ASC 310-30 [Member] | Non Owner-Occupied [Member]
Loans [Line Items]
Total Loans	5,722	4,430
Total Loans [Member] | Substandard [Member] | Non ASC 310-30 [Member] | Residential Real Estate Segment [Member]
Loans [Line Items]
Total Loans	6,289	7,089
Total Loans [Member] | Substandard [Member] | Non ASC 310-30 [Member] | Senior lien
Loans [Line Items]
Total Loans	4,921	5,744
Total Loans [Member] | Substandard [Member] | Non ASC 310-30 [Member] | Junior lien
Loans [Line Items]
Total Loans	1,368	1,345
Total Loans [Member] | Substandard [Member] | Non ASC 310-30 [Member] | Consumer [Member]
Loans [Line Items]
Total Loans	85	227
Total Loans [Member] | Substandard [Member] | ASC 310-30 [Member]
Loans [Line Items]
Total Loans	91,767	126,315
Total Loans [Member] | Substandard [Member] | ASC 310-30 [Member] | Commercial Loan [Member]
Loans [Line Items]
Total Loans	4,746	11,092
Total Loans [Member] | Substandard [Member] | ASC 310-30 [Member] | Agriculture [Member]
Loans [Line Items]
Total Loans	6,999	2,179
Total Loans [Member] | Substandard [Member] | ASC 310-30 [Member] | Residential Real Estate Segment [Member]
Loans [Line Items]
Total Loans	2,888	10,289
Total Loans [Member] | Substandard [Member] | ASC 310-30 [Member] | Consumer [Member]
Loans [Line Items]
Total Loans	341	789
Total Loans [Member] | Substandard [Member] | ASC 310-30 [Member] | Commercial Real Estate [Member]
Loans [Line Items]
Total Loans	76,793	101,966
Total Loans [Member] | Doubtful [Member]
Loans [Line Items]
Total Loans	8,113	4,761
Total Loans [Member] | Doubtful [Member] | Non ASC 310-30 [Member]
Loans [Line Items]
Total Loans	4,344	226
Total Loans [Member] | Doubtful [Member] | Non ASC 310-30 [Member] | Commercial Loan [Member]
Loans [Line Items]
Total Loans	4,044	80
Total Loans [Member] | Doubtful [Member] | Non ASC 310-30 [Member] | Total Commercial Real Estate [Member]
Loans [Line Items]
Total Loans	273	8
Total Loans [Member] | Doubtful [Member] | Non ASC 310-30 [Member] | Non Owner-Occupied [Member]
Loans [Line Items]
Total Loans	273	8
Total Loans [Member] | Doubtful [Member] | Non ASC 310-30 [Member] | Residential Real Estate Segment [Member]
Loans [Line Items]
Total Loans	27	138
Total Loans [Member] | Doubtful [Member] | Non ASC 310-30 [Member] | Senior lien
Loans [Line Items]
Total Loans	27	138
Total Loans [Member] | Doubtful [Member] | ASC 310-30 [Member]
Loans [Line Items]
Total Loans	3,769	4,535
Total Loans [Member] | Doubtful [Member] | ASC 310-30 [Member] | Commercial Loan [Member]
Loans [Line Items]
Total Loans		544
Total Loans [Member] | Doubtful [Member] | ASC 310-30 [Member] | Commercial Real Estate [Member]
Loans [Line Items]
Total Loans	$ 3,769	$ 3,991