Loans, Allowance for Loan Losses, and Credit Quality - Summary of Non-accretable discount on purchase credit impaired ("PCI") loans (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Accretion Discount [Roll Forward]
Balance at beginning of period	$ 6,408	$ 9,471	$ 9,471
Accretion to loan interest income	(3,293)	(1,872)	(1,976)
Transferred to accretable	0	(513)	(990)
Disposals of loans	(660)	(97)	(97)
Balance at end of period	$ 2,455	$ 6,989	$ 6,408