401(k) Plan 401(k) Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
401(k) Plan [Abstract]
Maximum contribution % of eligible pre-tax compensation allowed to participants of defined contribution plan	50.00%
Employer match	$ 7.0	$ 7.6	$ 7.2
Employer match, percent	50.00%
Employer match, percent of employees' compensation	6.00%
Percent of company stock purchased	6.00%	3.60%