Employee Benefit Plans (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) item	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($)
Employee Benefit Plans
Percentage of contributions of covered employees' salaries or wages	2.50%
Contribution	$ 14.2	$ 10.8	$ 7.8
Amount payable to plan	$ 0.3	$ 0.2
Number of employees who are union members | item	7	6
Contributions made to multi-employer pension plans	$ 0.0	$ 0.0	$ 0.0
Period in which certain individuals are entitled to fixed annual payments	15 years
Maximum age under which certain individuals are entitled to fixed annual payments	65 years
Portion of benefits vesting after ten years of completed service (as a percent)	50.00%
Period of completed service over which 50% of benefits are vested	10 years
Portion of benefits vesting after fifteen years of completed service (as a percent)	75.00%
Period of completed service over which 75% of benefits are vested	15 years
Period of service over which benefits are fully vested	20 years
Unfunded benefit liability	$ 4.1	$ 4.3