Discontinued Operations and Assets Held for Sale (Major Categories of Assets and Liabilities Reflected as Held for Sale) (Details) - USD ($) $ in Thousands		Jun. 30, 2019	Dec. 31, 2018
Current assets
Total current assets associated with assets held for sale		$ 1,863,825	$ 173,865	[1]
Non-current assets
Total non-current assets associated with assets held for sale	[1]	0	1,545,008
Current liabilities
Accounts payable		211,570	203,236
Other accrued liabilities		46,829	55,273
Current maturities of long-term debt		10,194	9,915
Taxes payable		2,340	18,034
Current operating lease liabilities		46,336	0
Long-term debt		112,680
Asset retirement obligation		280,408	0
Non-current operating lease liabilities		62,394	0
Total current liabilities associated with assets held for sale		772,751	286,458	[1]
Non-current liabilities
Long-term debt		0	117,816
Asset retirement obligation		0	274,904
Total non-current liabilities associated with assets held for sale	[1]	0	392,720
Discontinued Operations, Held-for-sale or Disposed of by Sale
Current assets
Cash		63,649	44,669
Accounts receivable		111,250	103,158
Inventories		8,652	7,887
Prepaid expenses and other		16,929	18,151
Property, Plant, and Equipment, net		1,355,229
Deferred income taxes and other assets		199,386
Operating lease asset		108,730
Total current assets associated with assets held for sale		1,863,825	173,865
Non-current assets
Property, Plant, and Equipment, net		0	1,325,431
Deferred income taxes and other assets		0	219,577
Operating lease asset		0	0
Total non-current assets associated with assets held for sale		$ 0	$ 1,545,008

[1] Reclassified to conform to current presentation (see Note A).