Lease Obligations (Operating Leases) (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Jan. 01, 2019
Operating Leased Assets [Line Items]
Operating leases, additions	$ 7,500	$ 54,100	$ 13,300	$ 66,400
Operating lease assets	198,844		198,844		$ 229,903
Operating leases, expense	26,800	27,400	62,100	55,300
Operating leases, variable lease costs	2,800	2,100	5,700	4,400
Operating leases, short-term leases, expense	$ 72,900	$ 115,600	$ 150,400	$ 208,400
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