Employee Benefit Plans (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)
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	$ 1.1	$ 0.8