Retirement Savings Plan (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Postemployment Benefits [Abstract]
Percentage of maximum limit for employees to contribute their cash compensation	75.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	4.00%
Total plan expenses		$ 7.3	$ 5.4	$ 2.9