Lease Obligations (Operating Leases) (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Jan. 01, 2019
Operating Leased Assets [Line Items]
Operating leases, additions	$ 3,300	$ 19,900	$ 16,600	$ 86,400
Operating lease assets	187,531		187,531		$ 229,903
Operating leases, expense	25,600	29,500	87,700	84,800
Operating leases, variable lease costs	2,300	3,100	8,000	7,500
Operating leases, short-term leases, expense	$ 74,000	$ 144,000	$ 224,400	$ 352,400
Minimum [Member]
Operating Leased Assets [Line Items]
Operating leases, term of contract (in years)	1 year		1 year
Maximum [Member]
Operating Leased Assets [Line Items]
Operating leases, term of contract (in years)	1 year		1 year
Accounting Standards Update 2016-02 [Member]
Operating Leased Assets [Line Items]
Operating lease assets						$ 230,000