Benefit Plans - Savings and Retirement Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Benefit Plans
Expenses related to the savings and retirement plan	$ 2.1	$ 2.1	$ 1.9
Savings And Retirement 401K Plan
Benefit Plans
Employer match of first tier of employee contributions (as a percent)	50.00%
First tier percentage of compensation eligible for match by employer	6.00%