Loans and Leases - Loan Acquired (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Loans and Leases Receivable Disclosure [Line Items]
Acquired loans, outstanding principal	$ 654,264	$ 833,352
Acquired loans, remaining loan mark	(24,949)	(34,790)
Acquired loans, recorded investment	629,315	798,562
Commercial mortgage
Loans and Leases Receivable Disclosure [Line Items]
Acquired loans, outstanding principal	369,894	412,263
Acquired loans, remaining loan mark	(10,798)	(11,213)
Acquired loans, recorded investment	359,096	401,050
Home equity loans and lines
Loans and Leases Receivable Disclosure [Line Items]
Acquired loans, outstanding principal	30,240	37,944
Acquired loans, remaining loan mark	(2,502)	(2,952)
Acquired loans, recorded investment	27,738	34,992
Residential mortgage
Loans and Leases Receivable Disclosure [Line Items]
Acquired loans, outstanding principal	89,302	101,523
Acquired loans, remaining loan mark	(2,937)	(3,572)
Acquired loans, recorded investment	86,365	97,951
Construction
Loans and Leases Receivable Disclosure [Line Items]
Acquired loans, outstanding principal	20,142	86,081
Acquired loans, remaining loan mark	(340)	(1,893)
Acquired loans, recorded investment	19,802	84,188
Commercial and industrial
Loans and Leases Receivable Disclosure [Line Items]
Acquired loans, outstanding principal	111,799	141,960
Acquired loans, remaining loan mark	(7,000)	(11,952)
Acquired loans, recorded investment	104,799	130,008
Consumer
Loans and Leases Receivable Disclosure [Line Items]
Acquired loans, outstanding principal	3,094	3,051
Acquired loans, remaining loan mark	(118)	(44)
Acquired loans, recorded investment	2,976	3,007
Leases
Loans and Leases Receivable Disclosure [Line Items]
Acquired loans, outstanding principal	29,793	50,530
Acquired loans, remaining loan mark	(1,254)	(3,164)
Acquired loans, recorded investment	$ 28,539	$ 47,366