Employee Savings Plan (Details) - USD ($) $ in Millions		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Plan Disclosure [Line Items]
Employer matching contribution (percent)		3.00%
Cost recognized		$ 2.9	$ 2.2	$ 1.7
Subsequent Event
Defined Contribution Plan Disclosure [Line Items]
Employer matching contribution (percent)	4.00%