Consolidated statements of comprehensive income (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net change in unrealized gain (loss) in available for sale securities, tax	$ 493	$ 5,309	$ (71)
Reclassification adjustment for gain on sale of securities included in net income, tax	112	$ 298	$ 91
Net change in unrealized gain in hedging activities, tax	$ 442