Long-Term Debt (GO Zone Bonds) (Details) - Senior Notes [Member] $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Nov. 30, 2017 USD ($)	Jul. 31, 2012 USD ($)	Dec. 31, 2010 USD ($)	Jul. 02, 2010 USD ($)
6 1/2% GO Zone Senior Notes Due 2035 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 89	$ 89			$ 89
Debt Instrument, Face Amount					$ 89
Stated interest rate (percent)	6.50%
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%
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%
Go Zone 3.50% Bonds Due 2032 [Member]
Debt Instrument [Line Items]
Amount of bonds remarketed			$ 250
Stated interest rate (percent)			3.50%
3.60% Senior Notes Due 2022 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 250	250		$ 250
Stated interest rate (percent)	3.60%
3.50% Senior Notes Due 2032 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 250	$ 250
Stated interest rate (percent)	3.50%