Goodwill and Core Deposit Intangibles - CDI Roll Forward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gross amount of CDI:
Balance, beginning of year	$ 15,102	$ 10,782	$ 7,876
Additions due to acquisitions	39,707	4,320	2,906
Balance, end of year	54,809	15,102	10,782
Accumulated amortization:
Balance, beginning of year	(5,651)	(3,612)	(2,262)
Amortization	(6,144)	(2,039)	(1,350)
Balance, end of year	(11,795)	(5,651)	(3,612)
Net CDI, end of year	$ 43,014	$ 9,451	$ 7,170