Quarterly Financial Data (Unaudited) (Parenthetical) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Impact of enactment of federal tax reform	$ 1,800	$ 1,793	$ 0	$ 0