Allowance for Loan Losses and Credit Quality of Loans, Credit Quality by Loan Class (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Net loans	$ 6,466,934	$ 6,198,057
Commercial Credit Exposure [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	2,962,287	2,786,002
Consumer Credit Exposure [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	2,202,070	2,149,441
Residential Mortgage Credit Exposure [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	1,302,577	1,262,614
Originated Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	6,045,111	5,697,963
Originated Loans [Member] | Commercial Credit Exposure [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	2,765,843	2,548,384
Originated Loans [Member] | Commercial Credit Exposure [Member] | Total Commercial and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	2,283,911	2,075,697
Originated Loans [Member] | Commercial Credit Exposure [Member] | Total Commercial and Agricultural [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	2,158,075	1,970,912
Originated Loans [Member] | Commercial Credit Exposure [Member] | Total Commercial and Agricultural [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	76,284	40,724
Originated Loans [Member] | Commercial Credit Exposure [Member] | Total Commercial and Agricultural [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	49,552	64,056
Originated Loans [Member] | Commercial Credit Exposure [Member] | Total Commercial and Agricultural [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	0	5
Originated Loans [Member] | Commercial Credit Exposure [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	733,122	653,570
Originated Loans [Member] | Commercial Credit Exposure [Member] | Commercial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	677,943	616,829
Originated Loans [Member] | Commercial Credit Exposure [Member] | Commercial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	40,123	7,750
Originated Loans [Member] | Commercial Credit Exposure [Member] | Commercial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	15,056	28,991
Originated Loans [Member] | Commercial Credit Exposure [Member] | Commercial [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	0	0
Originated Loans [Member] | Commercial Credit Exposure [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	1,480,559	1,351,789
Originated Loans [Member] | Commercial Credit Exposure [Member] | Commercial Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	1,424,353	1,288,409
Originated Loans [Member] | Commercial Credit Exposure [Member] | Commercial Real Estate [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	29,785	31,053
Originated Loans [Member] | Commercial Credit Exposure [Member] | Commercial Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	26,421	32,327
Originated Loans [Member] | Commercial Credit Exposure [Member] | Commercial Real Estate [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	0	0
Originated Loans [Member] | Commercial Credit Exposure [Member] | Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	34,839	37,916
Originated Loans [Member] | Commercial Credit Exposure [Member] | Agricultural [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	30,233	36,762
Originated Loans [Member] | Commercial Credit Exposure [Member] | Agricultural [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	3,033	25
Originated Loans [Member] | Commercial Credit Exposure [Member] | Agricultural [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	1,573	1,124
Originated Loans [Member] | Commercial Credit Exposure [Member] | Agricultural [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	0	5
Originated Loans [Member] | Commercial Credit Exposure [Member] | Agricultural Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	35,391	32,422
Originated Loans [Member] | Commercial Credit Exposure [Member] | Agricultural Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	25,546	28,912
Originated Loans [Member] | Commercial Credit Exposure [Member] | Agricultural Real Estate [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	3,343	1,896
Originated Loans [Member] | Commercial Credit Exposure [Member] | Agricultural Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	6,502	1,614
Originated Loans [Member] | Commercial Credit Exposure [Member] | Agricultural Real Estate [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	0	0
Originated Loans [Member] | Commercial Credit Exposure [Member] | Business Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	481,932	472,687
Originated Loans [Member] | Business Banking Credit Exposure [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	481,932	472,687
Originated Loans [Member] | Business Banking Credit Exposure [Member] | Non-classified [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	469,025	458,864
Originated Loans [Member] | Business Banking Credit Exposure [Member] | Classified [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	12,907	13,823
Originated Loans [Member] | Business Banking Credit Exposure [Member] | Business Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	481,932	472,687
Originated Loans [Member] | Business Banking Credit Exposure [Member] | Business Banking [Member] | Non-classified [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	469,025	458,864
Originated Loans [Member] | Business Banking Credit Exposure [Member] | Business Banking [Member] | Classified [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	12,907	13,823
Originated Loans [Member] | Consumer Credit Exposure [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	2,154,084	2,086,436
Originated Loans [Member] | Consumer Credit Exposure [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	2,146,403	2,078,286
Originated Loans [Member] | Consumer Credit Exposure [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	7,681	8,150
Originated Loans [Member] | Consumer Credit Exposure [Member] | Indirect [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	1,631,278	1,566,675
Originated Loans [Member] | Consumer Credit Exposure [Member] | Indirect [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	1,626,757	1,562,031
Originated Loans [Member] | Consumer Credit Exposure [Member] | Indirect [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	4,521	4,644
Originated Loans [Member] | Consumer Credit Exposure [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	458,753	456,657
Originated Loans [Member] | Consumer Credit Exposure [Member] | Home Equity [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	455,750	453,278
Originated Loans [Member] | Consumer Credit Exposure [Member] | Home Equity [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	3,003	3,379
Originated Loans [Member] | Consumer Credit Exposure [Member] | Direct [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	64,053	63,104
Originated Loans [Member] | Consumer Credit Exposure [Member] | Direct [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	63,896	62,977
Originated Loans [Member] | Consumer Credit Exposure [Member] | Direct [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	157	127
Originated Loans [Member] | Residential Mortgage Credit Exposure [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	1,125,184	1,063,143
Originated Loans [Member] | Residential Mortgage Credit Exposure [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	1,118,407	1,055,055
Originated Loans [Member] | Residential Mortgage Credit Exposure [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	6,777	8,088
Originated Loans [Member] | Residential Mortgage Credit Exposure [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	1,125,184	1,063,143
Originated Loans [Member] | Residential Mortgage Credit Exposure [Member] | Residential Mortgage [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	1,118,407	1,055,055
Originated Loans [Member] | Residential Mortgage Credit Exposure [Member] | Residential Mortgage [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	6,777	8,088
Acquired Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	421,823	500,094
Acquired Loans [Member] | Commercial Credit Exposure [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	196,444	237,618
Acquired Loans [Member] | Commercial Credit Exposure [Member] | Total Commercial and Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	152,348	186,736
Acquired Loans [Member] | Commercial Credit Exposure [Member] | Total Commercial and Agricultural [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	143,532	175,854
Acquired Loans [Member] | Commercial Credit Exposure [Member] | Total Commercial and Agricultural [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	839	1,307
Acquired Loans [Member] | Commercial Credit Exposure [Member] | Total Commercial and Agricultural [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	7,977	8,370
Acquired Loans [Member] | Commercial Credit Exposure [Member] | Total Commercial and Agricultural [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans		1,205
Acquired Loans [Member] | Commercial Credit Exposure [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	39,037	49,447
Acquired Loans [Member] | Commercial Credit Exposure [Member] | Commercial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	37,271	48,194
Acquired Loans [Member] | Commercial Credit Exposure [Member] | Commercial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	327	76
Acquired Loans [Member] | Commercial Credit Exposure [Member] | Commercial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	1,439	1,177
Acquired Loans [Member] | Commercial Credit Exposure [Member] | Commercial [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans		0
Acquired Loans [Member] | Commercial Credit Exposure [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	113,311	137,289
Acquired Loans [Member] | Commercial Credit Exposure [Member] | Commercial Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	106,261	127,660
Acquired Loans [Member] | Commercial Credit Exposure [Member] | Commercial Real Estate [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	512	1,231
Acquired Loans [Member] | Commercial Credit Exposure [Member] | Commercial Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	6,538	7,193
Acquired Loans [Member] | Commercial Credit Exposure [Member] | Commercial Real Estate [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans		1,205
Acquired Loans [Member] | Commercial Credit Exposure [Member] | Business Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	44,096	50,882
Acquired Loans [Member] | Business Banking Credit Exposure [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	44,096	50,882
Acquired Loans [Member] | Business Banking Credit Exposure [Member] | Non-classified [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	40,636	47,347
Acquired Loans [Member] | Business Banking Credit Exposure [Member] | Classified [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	3,460	3,535
Acquired Loans [Member] | Business Banking Credit Exposure [Member] | Business Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	44,096	50,882
Acquired Loans [Member] | Business Banking Credit Exposure [Member] | Business Banking [Member] | Non-classified [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	40,636	47,347
Acquired Loans [Member] | Business Banking Credit Exposure [Member] | Business Banking [Member] | Classified [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	3,460	3,535
Acquired Loans [Member] | Consumer Credit Exposure [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	47,986	63,005
Acquired Loans [Member] | Consumer Credit Exposure [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	47,778	62,671
Acquired Loans [Member] | Consumer Credit Exposure [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	208	334
Acquired Loans [Member] | Consumer Credit Exposure [Member] | Indirect [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	2,154	8,693
Acquired Loans [Member] | Consumer Credit Exposure [Member] | Indirect [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	2,121	8,646
Acquired Loans [Member] | Consumer Credit Exposure [Member] | Indirect [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	33	47
Acquired Loans [Member] | Consumer Credit Exposure [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	42,977	51,127
Acquired Loans [Member] | Consumer Credit Exposure [Member] | Home Equity [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	42,817	50,860
Acquired Loans [Member] | Consumer Credit Exposure [Member] | Home Equity [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	160	267
Acquired Loans [Member] | Consumer Credit Exposure [Member] | Direct [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	2,855	3,185
Acquired Loans [Member] | Consumer Credit Exposure [Member] | Direct [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	2,840	3,165
Acquired Loans [Member] | Consumer Credit Exposure [Member] | Direct [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	15	20
Acquired Loans [Member] | Residential Mortgage Credit Exposure [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	177,393	199,471
Acquired Loans [Member] | Residential Mortgage Credit Exposure [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	175,631	196,508
Acquired Loans [Member] | Residential Mortgage Credit Exposure [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	1,762	2,963
Acquired Loans [Member] | Residential Mortgage Credit Exposure [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	177,393	199,471
Acquired Loans [Member] | Residential Mortgage Credit Exposure [Member] | Residential Mortgage [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	175,631	196,508
Acquired Loans [Member] | Residential Mortgage Credit Exposure [Member] | Residential Mortgage [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Net loans	$ 1,762	$ 2,963