Loans and Leases - Allowance for Loan and Lease Losses by Method of Impairment (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Allowance on loans and leases:
Individually evaluated for impairment	$ 278		$ 258
Collectively evaluated for impairment	18,406		17,267
Allowance on loans and leases	18,684	$ 19,398	17,525	$ 17,004	$ 16,399	$ 17,486
Carrying value of loans and leases:
Individually evaluated for impairment	11,470		13,940
Collectively evaluated for impairment	3,342,486		3,241,069
Portfolio loans and leases	3,381,475		3,285,858
Purchased credit-impaired
Allowance on loans and leases:
Allowance on loans and leases	0		0
Carrying value of loans and leases:
Portfolio loans and leases	27,519		30,849
Originated
Allowance on loans and leases:
Individually evaluated for impairment	206		208
Collectively evaluated for impairment	18,406		17,267
Allowance on loans and leases	18,612		17,475
Carrying value of loans and leases:
Individually evaluated for impairment	8,282		8,878
Collectively evaluated for impairment	2,743,878		2,478,418
Portfolio loans and leases	2,752,160		2,487,296
Acquired
Carrying value of loans and leases:
Individually evaluated for impairment	3,188		5,062
Collectively evaluated for impairment	598,608		762,651
Portfolio loans and leases	629,315		798,562
Financial Asset Acquired With and Without Credit Deterioration
Allowance on loans and leases:
Individually evaluated for impairment	72		50
Collectively evaluated for impairment	0		0
Allowance on loans and leases	72		50
Commercial mortgage
Allowance on loans and leases:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	7,544		7,550
Allowance on loans and leases	7,544	8,033	7,550	7,332	6,608	6,227
Carrying value of loans and leases:
Individually evaluated for impairment	736		2,128
Collectively evaluated for impairment	1,609,375		1,503,825
Portfolio loans and leases	1,618,493		1,523,377
Commercial mortgage | Purchased credit-impaired
Allowance on loans and leases:
Allowance on loans and leases	0		0
Carrying value of loans and leases:
Portfolio loans and leases	8,382		17,424
Commercial mortgage | Originated
Allowance on loans and leases:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	7,544		7,550
Allowance on loans and leases	7,544		7,550
Carrying value of loans and leases:
Individually evaluated for impairment	0		1,345
Collectively evaluated for impairment	1,259,397		1,120,982
Portfolio loans and leases	1,259,397		1,122,327
Commercial mortgage | Acquired
Carrying value of loans and leases:
Individually evaluated for impairment	736		783
Collectively evaluated for impairment	349,978		382,843
Portfolio loans and leases	359,096		401,050
Commercial mortgage | Financial Asset Acquired With and Without Credit Deterioration
Allowance on loans and leases:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	0		0
Allowance on loans and leases	0		0
Home equity loans and lines
Allowance on loans and leases:
Individually evaluated for impairment	19		19
Collectively evaluated for impairment	873		1,067
Allowance on loans and leases	892	933	1,086	1,092	1,214	1,255
Carrying value of loans and leases:
Individually evaluated for impairment	2,602		2,162
Collectively evaluated for impairment	204,694		215,604
Portfolio loans and leases	207,806		218,275
Home equity loans and lines | Purchased credit-impaired
Allowance on loans and leases:
Allowance on loans and leases	0		0
Carrying value of loans and leases:
Portfolio loans and leases	510		509
Home equity loans and lines | Originated
Allowance on loans and leases:
Individually evaluated for impairment	19		19
Collectively evaluated for impairment	873		1,067
Allowance on loans and leases	892		1,086
Carrying value of loans and leases:
Individually evaluated for impairment	2,361		1,902
Collectively evaluated for impairment	177,707		181,381
Portfolio loans and leases	180,068		183,283
Home equity loans and lines | Acquired
Carrying value of loans and leases:
Individually evaluated for impairment	241		260
Collectively evaluated for impairment	26,987		34,223
Portfolio loans and leases	27,738		34,992
Home equity loans and lines | Financial Asset Acquired With and Without Credit Deterioration
Allowance on loans and leases:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	0		0
Allowance on loans and leases	0		0
Residential mortgage
Allowance on loans and leases:
Individually evaluated for impairment	228		230
Collectively evaluated for impairment	1,590		1,696
Allowance on loans and leases	1,818	1,933	1,926	1,821	1,776	1,917
Carrying value of loans and leases:
Individually evaluated for impairment	5,940		7,726
Collectively evaluated for impairment	461,462		451,160
Portfolio loans and leases	467,402		458,886
Residential mortgage | Purchased credit-impaired
Allowance on loans and leases:
Allowance on loans and leases	0		0
Carrying value of loans and leases:
Portfolio loans and leases	0		0
Residential mortgage | Originated
Allowance on loans and leases:
Individually evaluated for impairment	179		180
Collectively evaluated for impairment	1,590		1,696
Allowance on loans and leases	1,769		1,876
Carrying value of loans and leases:
Individually evaluated for impairment	4,268		4,418
Collectively evaluated for impairment	376,769		356,517
Portfolio loans and leases	381,037		360,935
Residential mortgage | Acquired
Carrying value of loans and leases:
Individually evaluated for impairment	1,672		3,308
Collectively evaluated for impairment	84,693		94,643
Portfolio loans and leases	86,365		97,951
Residential mortgage | Financial Asset Acquired With and Without Credit Deterioration
Allowance on loans and leases:
Individually evaluated for impairment	49		50
Collectively evaluated for impairment	0		0
Allowance on loans and leases	49		50
Construction
Allowance on loans and leases:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	1,241		937
Allowance on loans and leases	1,241	1,158	937	930	1,111	2,233
Carrying value of loans and leases:
Individually evaluated for impairment	291		0
Collectively evaluated for impairment	175,802		204,088
Portfolio loans and leases	178,493		212,454
Construction | Purchased credit-impaired
Allowance on loans and leases:
Allowance on loans and leases	0		0
Carrying value of loans and leases:
Portfolio loans and leases	2,400		8,366
Construction | Originated
Allowance on loans and leases:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	1,241		937
Allowance on loans and leases	1,241		937
Carrying value of loans and leases:
Individually evaluated for impairment	291		0
Collectively evaluated for impairment	158,400		128,266
Portfolio loans and leases	158,691		128,266
Construction | Acquired
Carrying value of loans and leases:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	17,402		75,822
Portfolio loans and leases	19,802		84,188
Construction | Financial Asset Acquired With and Without Credit Deterioration
Allowance on loans and leases:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	0		0
Allowance on loans and leases	0		0
Commercial and industrial
Allowance on loans and leases:
Individually evaluated for impairment	11		5
Collectively evaluated for impairment	5,294		5,033
Allowance on loans and leases	5,305	5,672	5,038	4,880	4,813	5,142
Carrying value of loans and leases:
Individually evaluated for impairment	1,757		1,897
Collectively evaluated for impairment	705,015		712,865
Portfolio loans and leases	722,999		719,312
Commercial and industrial | Purchased credit-impaired
Allowance on loans and leases:
Allowance on loans and leases	0		0
Carrying value of loans and leases:
Portfolio loans and leases	16,227		4,550
Commercial and industrial | Originated
Allowance on loans and leases:
Individually evaluated for impairment	4		5
Collectively evaluated for impairment	5,294		5,033
Allowance on loans and leases	5,298		5,038
Carrying value of loans and leases:
Individually evaluated for impairment	1,294		1,186
Collectively evaluated for impairment	616,906		588,118
Portfolio loans and leases	618,200		589,304
Commercial and industrial | Acquired
Carrying value of loans and leases:
Individually evaluated for impairment	463		711
Collectively evaluated for impairment	88,109		124,747
Portfolio loans and leases	104,799		130,008
Commercial and industrial | Financial Asset Acquired With and Without Credit Deterioration
Allowance on loans and leases:
Individually evaluated for impairment	7		0
Collectively evaluated for impairment	0		0
Allowance on loans and leases	7		0
Consumer
Allowance on loans and leases:
Individually evaluated for impairment	20		4
Collectively evaluated for impairment	298		242
Allowance on loans and leases	318	289	246	210	177	153
Carrying value of loans and leases:
Individually evaluated for impairment	144		27
Collectively evaluated for impairment	47,665		38,126
Portfolio loans and leases	47,809		38,153
Consumer | Purchased credit-impaired
Allowance on loans and leases:
Allowance on loans and leases	0		0
Carrying value of loans and leases:
Portfolio loans and leases	0		0
Consumer | Originated
Allowance on loans and leases:
Individually evaluated for impairment	4		4
Collectively evaluated for impairment	298		242
Allowance on loans and leases	302		246
Carrying value of loans and leases:
Individually evaluated for impairment	68		27
Collectively evaluated for impairment	44,765		35,119
Portfolio loans and leases	44,833		35,146
Consumer | Acquired
Carrying value of loans and leases:
Individually evaluated for impairment	76		0
Collectively evaluated for impairment	2,900		3,007
Portfolio loans and leases	2,976		3,007
Consumer | Financial Asset Acquired With and Without Credit Deterioration
Allowance on loans and leases:
Individually evaluated for impairment	16		0
Collectively evaluated for impairment	0		0
Allowance on loans and leases	16		0
Leases
Allowance on loans and leases:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	1,566		742
Allowance on loans and leases	1,566	1,380	742	739	700	559
Carrying value of loans and leases:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	138,473		115,401
Portfolio loans and leases	138,473		115,401
Leases | Purchased credit-impaired
Allowance on loans and leases:
Allowance on loans and leases	0		0
Carrying value of loans and leases:
Portfolio loans and leases	0		0
Leases | Originated
Allowance on loans and leases:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	1,566		742
Allowance on loans and leases	1,566		742
Carrying value of loans and leases:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	109,934		68,035
Portfolio loans and leases	109,934		68,035
Leases | Acquired
Carrying value of loans and leases:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	28,539		47,366
Portfolio loans and leases	28,539		47,366
Leases | Financial Asset Acquired With and Without Credit Deterioration
Allowance on loans and leases:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	0		0
Allowance on loans and leases	0		0
Unallocated
Allowance on loans and leases:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	0		0
Allowance on loans and leases	0	$ 0	0	$ 0	$ 0	$ 0
Unallocated | Purchased credit-impaired
Allowance on loans and leases:
Allowance on loans and leases	$ 0		$ 0