Lease Obligations (Operating Leases) (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2019	Dec. 31, 2018
Operating Leased Assets [Line Items]
Operating lease assets	$ 223,106			$ 0
Operating leases, additions	12,000
Operating leases, variable cost	2,300
Operating leases, short-term lease, expense	92,800
Lease Term in Excess of One Year [Member]
Operating Leased Assets [Line Items]
Operating leases, expense	$ 27,900
Operating leases, rent expense		$ 26,900
Lease Term in Excess of One Year [Member] | Minimum [Member]
Operating Leased Assets [Line Items]
Operating leases, term of contract (in years)	1 year	1 year
Lease Term of One Year or Less [Member]
Operating Leased Assets [Line Items]
Operating leases, rent expense		$ 91,700
Lease Term of One Year or Less [Member] | Maximum [Member]
Operating Leased Assets [Line Items]
Operating leases, term of contract (in years)	1 year	1 year
ASU 2016-02 [Member]
Operating Leased Assets [Line Items]
Operating lease assets			$ 233,000