Summary of Significant Accounting Policies and Other Items - Employee Benefit Plans (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Summary of Significant Accounting Policies and Other Items
Employer match of first tier of employee contribution (as a percent)	100	100	100
First tier percentage of compensation eligible for match	3	3	3
Employer match of second tier of employee contribution (as a percent)	50	50	50
Second tier percentage of compensation eligible for match	4	4	4
Employer matching contributions	$ 3.1	$ 3.7	$ 2.2
Employer non-elective contribution	$ 0.1	$ 0.0	$ 0.0