Goodwill and Acquired Intangible Assets - Gross Carrying Amount and Accumulated Amortization of Intangible Asset (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]
Remaining Average Amortization Period for CDI	63 months	64 months	67 months
Beginning of year	$ 14,572	$ 8,594	$ 7,454
Acquired CDI	7,726	8,464	2,564
Amortization expense	(3,361)	(2,486)	(1,424)
End of year	$ 19,099	$ 14,572	$ 8,594