Summary of Significant Accounting Policies (Other Assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Restricted assets	$ 3,456	$ 5,241
Prepaid pension costs	35,209	31,686
Contract assets, net	24,002	18,443
Other assets	32,359	48,778
Total other assets	$ 95,026	$ 104,148