Goodwill - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended	12 Months Ended
	Sep. 30, 2017	Mar. 31, 2016	Sep. 30, 2017	Dec. 31, 2016	Jul. 14, 2017	May 19, 2017	Dec. 31, 2015
Goodwill [Line Items]
Goodwill	$ 445,355		$ 445,355	$ 244,000			$ 243,129
Goodwill, measurement period adjustment			$ (469)	$ 871
IAB
Goodwill [Line Items]
Goodwill					$ 153,636
Arlington Bank
Goodwill [Line Items]
Goodwill						$ 47,719
Goodwill, measurement period adjustment	$ 469
Ameriana
Goodwill [Line Items]
Goodwill							$ 38,624
Goodwill, measurement period adjustment		$ 871