Employee Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee Benefits
Company matching contribution to defined contribution plans, percentage of employee contribution	100.00%
Company's matching contribution as percentage of employee salary, maximum	4.00%
Company's matching contribution vesting percentage, per year	20.00%
Company's contribution to defined contribution benefit plans	$ 3.7	$ 2.9	$ 2.4