Summary of Significant Accounting Policies (Unfulfilled Performance Obligations) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 631.2	$ 806.9
Revenue, Remaining Performance Obligation, Amount, Current	$ 449.4