Description of the Business and Significant Accounting Policies (Cash and Equivalents, and Restricted Cash and Equivalents) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Description of the Business and Significant Accounting Policies
Restricted Cash and Cash Equivalents	$ 5.5	$ 5.5	$ 5.5	$ 5.9
Restricted Cash and Cash Equivalents, Asset, Statement of Financial Position [Extensible List]	Other assets	Other assets	Other assets	Other assets