Long-Term Debt (GO Zone Bonds) (Details) $ in Millions		12 Months Ended
	Nov. 01, 2017 USD ($)	Dec. 31, 2017 USD ($)	Nov. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2010 USD ($)	Jul. 02, 2010 USD ($)	Dec. 13, 2007 USD ($)
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 3,853		$ 3,678
Senior Notes [Member]
Debt Instrument [Line Items]
Minimum Debt Amount Guaranteed By Subsidiaries		5
Senior Notes [Member] | 6 1/2% GO Zone Tax-Exempt Revenue Bonds Due 2035 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 89		89	$ 89
Debt Instrument, Face Amount							$ 250
Stated interest rate (percent)		6.50%
Senior Notes [Member] | 6.75% Go Zone Senior Notes Due 2032 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 0		250
Debt Instrument, Face Amount					$ 89
Extinguishment of Debt, Amount	$ 250
Stated interest rate (percent)		6.75%
Senior Notes [Member] | Go Zone 3.50% Bonds Due 2032 [Member]
Debt Instrument [Line Items]
Amount of bonds remarketed			$ 250
Stated interest rate (percent)			3.50%
Senior Notes [Member] | 6 1/2% GO Zone Senior Notes Due 2029 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 100		100
Debt Instrument, Face Amount						$ 100
Stated interest rate (percent)		6.50%
Senior Notes [Member] | 3.50% Senior Notes Due 2032 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 250		0
Stated interest rate (percent)		3.50%
Minimum Debt Amount Guaranteed By Subsidiaries		$ 40
Fixed Charge Coverage Ratio		2.0
Maximum Unrestricted Quarterly Dividends Allowable Under Debt Covenants		0.10
Maximum Unrestricted Distributions Allowable Under Debt Covenants		$ 100
Maximum Percentage Of Income Distribution		50.00%
Maximum Distribution Percentage Of Specified Net Cash Proceeds		100.00%
Senior Notes [Member] | Waste Disposal Revenue Bonds Due 2027 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 11		$ 11