Supplemental Cash Flow Information (Reconciliation of Cash, Cash Equivalents and Restricted Cash to Statement of Cash Flows) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Supplemental Cash Flow Information [Abstract]
Cash and cash equivalents	$ 325,833	$ 102,580	$ 186,683
Restricted cash	3,793	5,728	17,671
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$ 329,626	$ 108,308	$ 204,354	$ 54,948