Loans, Allowance for Credit Losses - Loans, 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	Dec. 31, 2019
Accretion Discount [Roll Forward]
Balance at beginning of period	$ 6,408	$ 6,408
Acquired balance, net	0	911
Accretion to loan interest income	(221)	(4,713)
Disposals of loans	0	(679)
Balance at end of period	$ 6,187	$ 1,927