Loans and Loans Held for Sale - Summary of Restructured Loans (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Financing Receivable, Modifications [Line Items]
Restructured loans	[1]	$ 633	$ 1,471	$ 11,699	$ 4,522
Commercial real estate
Financing Receivable, Modifications [Line Items]
Restructured loans	[1]	250	400	250	500
Commercial and industrial
Financing Receivable, Modifications [Line Items]
Restructured loans	[1]	0	816	10,348	3,044
Residential mortgage
Financing Receivable, Modifications [Line Items]
Restructured loans	[1]	186	0	380	32
Home equity
Financing Receivable, Modifications [Line Items]
Restructured loans	[1]	191	70	715	726
Installment and other consumer
Financing Receivable, Modifications [Line Items]
Restructured loans	[1]	$ 6	$ 185	6	$ 220
Performing TDRs
Financing Receivable, Modifications [Line Items]
Restructured loans				22,226		$ 14,901
Performing TDRs | Commercial real estate
Financing Receivable, Modifications [Line Items]
Restructured loans				2,095		2,579
Performing TDRs | Commercial and industrial
Financing Receivable, Modifications [Line Items]
Restructured loans				11,874		3,946
Performing TDRs | Commercial construction
Financing Receivable, Modifications [Line Items]
Restructured loans				2,400		2,420
Performing TDRs | Residential mortgage
Financing Receivable, Modifications [Line Items]
Restructured loans				2,229		2,039
Performing TDRs | Home equity
Financing Receivable, Modifications [Line Items]
Restructured loans				3,612		3,885
Performing TDRs | Installment and other consumer
Financing Receivable, Modifications [Line Items]
Restructured loans				16		32
Nonperforming TDRs
Financing Receivable, Modifications [Line Items]
Restructured loans				7,143		11,150
Nonperforming TDRs | Commercial real estate
Financing Receivable, Modifications [Line Items]
Restructured loans				1,152		967
Nonperforming TDRs | Commercial and industrial
Financing Receivable, Modifications [Line Items]
Restructured loans				2,260		3,197
Nonperforming TDRs | Commercial construction
Financing Receivable, Modifications [Line Items]
Restructured loans				408		2,413
Nonperforming TDRs | Residential mortgage
Financing Receivable, Modifications [Line Items]
Restructured loans				1,913		3,585
Nonperforming TDRs | Home equity
Financing Receivable, Modifications [Line Items]
Restructured loans				1,404		979
Nonperforming TDRs | Installment and other consumer
Financing Receivable, Modifications [Line Items]
Restructured loans				6		9
Total TDRs
Financing Receivable, Modifications [Line Items]
Restructured loans				29,369		26,051
Total TDRs | Commercial real estate
Financing Receivable, Modifications [Line Items]
Restructured loans				3,247		3,546
Total TDRs | Commercial and industrial
Financing Receivable, Modifications [Line Items]
Restructured loans				14,134		7,143
Total TDRs | Commercial construction
Financing Receivable, Modifications [Line Items]
Restructured loans				2,808		4,833
Total TDRs | Residential mortgage
Financing Receivable, Modifications [Line Items]
Restructured loans				4,142		5,624
Total TDRs | Home equity
Financing Receivable, Modifications [Line Items]
Restructured loans				5,016		4,864
Total TDRs | Installment and other consumer
Financing Receivable, Modifications [Line Items]
Restructured loans				$ 22		$ 41

[1] Excludes loans that were fully paid off or fully charged-off by period end. The pre-modification balance represents the balance outstanding prior to modification. The post-modification balance represents the outstanding balance at period end.