Lease Obligations (Operating Leases) (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jan. 01, 2019	Dec. 31, 2018
Operating Leased Assets [Line Items]
Operating lease assets	$ 241,493		$ 241,493			$ 0
Operating leases, additions	54,000		66,000
Operating leases, variable lease costs	2,100		4,400
Operating leases, short-term leases, expense	115,600		208,400
Lease Term in Excess of One Year [Member]
Operating Leased Assets [Line Items]
Operating leases, expense	$ 27,400		$ 55,300
Operating leases, rent and related expense		$ 25,300		$ 52,200
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	1 year	1 year
Lease Term of One Year or Less [Member]
Operating Leased Assets [Line Items]
Operating leases, rent and related expense		$ 104,900		$ 196,600
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	1 year	1 year
ASU 2016-02 [Member]
Operating Leased Assets [Line Items]
Operating lease assets					$ 230,000