Employee Benefit Plans (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)
Employee Benefit Plans
Employer match of first tier of employee contribution (as a percent)			100
First tier percentage of compensation eligible for match			3
Employer match of second tier of employee contribution (as a percent)			50
Second tier percentage of compensation eligible for match			4
Employer matching contributions	$ 0.9	$ 0.5	$ 1.7	$ 0.9