Lease Obligations (Operating Leases) (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2019
Operating Leased Assets [Line Items]
Operating leases, additions	$ 28,000	$ 103,300
Operating lease assets	176,573	229,903
Operating leases, expense	113,000	114,500
Operating leases, variable lease costs	10,000	10,400
Operating leases, short-term leases, expense	$ 312,000	$ 448,200
Minimum [Member]
Operating Leased Assets [Line Items]
Operating leases, term of contract (in years)	1 year
Maximum [Member]
Operating Leased Assets [Line Items]
Operating leases, term of contract (in years)	1 year
Lease Terms in Excess of One Year [Member]
Operating Leased Assets [Line Items]
Operating leases, rent and related expense			$ 115,000
Lease Term Less Than One Year [Member]
Operating Leased Assets [Line Items]
Operating leases, rent and related expense			$ 472,100
ASU 2016-02 [Member]
Operating Leased Assets [Line Items]
Operating lease assets				$ 230,000