Loans, Allowance for Loan Losses, and Credit Quality - Summary of Non-accretable discount on purchase credit impaired ("PCI") loans (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Accretion Discount [Roll Forward]
Balance at beginning of period	$ 6,408	$ 9,471	$ 9,471
Accretion to loan interest income	(221)	(1,544)	(1,976)
Transferred to accretable	0	(55)	(990)
Disposals of loans	0	0	(97)
Balance at end of period	$ 6,187	$ 7,872	$ 6,408