Purchased Loans - Changes in Restructured Loans (Details) - Purchased $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) loan	Dec. 31, 2016 USD ($) loan
Number of Loans
Beginning balance (loans) | loan	42	26
Additional loans with concessions (loans) | loan	36	25
Reclassified as nonperforming (loans) | loan	(10)	(4)
Paid in full (loans) | loan	(3)	(5)
Charge-offs (loans) | loan	(1)
Principal paydowns (loans) | loan	0	0
Lapse of concession period (loans) | loan	(1)
Reclassified as performing (loans) | loan	(9)
Ending balance (loans) | loan	72	42
Recorded Investment
Beginning balance | $	$ 4,028	$ 3,201
Additional loans with concessions | $	5,703	2,472
Reclassified as nonperforming | $	(786)	(216)
Paid in full | $	(323)	(1,297)
Charge-offs | $	(17)
Principal paydowns | $	(377)	(132)
Lapse of concession period | $	(101)
Reclassified as performing | $	(838)
Ending balance | $	$ 8,965	$ 4,028