Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Benefit Plans
Maximum contribution to defined contribution retirement plan by employee, as a percentage of compensation	75.00%
Employer's matching contribution as a percentage of employee's contribution	50.00%
Maximum employer matching contribution to defined contribution retirement plan, as a percentage of compensation	6.00%
Expense recognized	$ 18.1	$ 15.2	$ 14.9