Summary of Significant Accounting Policies (Other Assets) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Accounting Policies [Abstract]
Restricted assets	$ 3,800	$ 3,456
Prepaid pension costs	36,022	35,209
Contract assets, net	28,688	24,002
PropTech investments at cost	3,563	0
Other assets	34,694	32,359
Total other assets	$ 106,767	$ 95,026