Lease Accounting (Lease Balances by Major Categories) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Operating Leased Assets [Line Items]
Total Right of Use Assets, net	$ 153,925	$ 177,071
Accrued liabilities, current	23,348	28,406
Other long-term liabilities, non-current	131,623	144,910
Present value of operating lease liabilities	$ 154,971	$ 173,316
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:AccruedLiabilitiesMember	us-gaap:AccruedLiabilitiesMember
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherNoncurrentLiabilitiesMember	us-gaap:OtherNoncurrentLiabilitiesMember
Transportation equipment
Operating Leased Assets [Line Items]
Total Right of Use Assets, net	$ 88,038	$ 98,588
Present value of operating lease liabilities	89,842
Office Space and Equipment
Operating Leased Assets [Line Items]
Total Right of Use Assets, net	7,489	11,085
Present value of operating lease liabilities	7,661
Facilities and Equipment
Operating Leased Assets [Line Items]
Total Right of Use Assets, net	58,398	$ 67,398
Present value of operating lease liabilities	$ 57,468