Lease Obligations (Operating Leases) (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Jan. 01, 2019
Operating Leased Assets [Line Items]
Operating leases, additions	$ 5,800	$ 12,000
Operating lease assets	210,637		$ 229,903
Operating leases, expense	35,300	27,900
Operating leases, variable lease costs	2,900	2,300
Operating leases, short-term leases, expense	$ 77,500	$ 92,800
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
Accounting Standards Update 2016-02 [Member]
Operating Leased Assets [Line Items]
Operating lease assets				$ 230,000