Employee Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Maximum percentage of annual compensation contributed by the participant (percent)	60.00%
Company contribution	$ 12.3	$ 11.8	$ 12.8
Common stock shares remained available for grant (shares)	754,744