Available-for-Sale Securities (Details) - USD ($)		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Amortized cost - maturity less than 1 year		$ 10,000,000		$ 10,000,000
Amortized cost - maturity between 1 and 5 years		82,000,000		82,000,000
Amortized cost - maturity between 6 and 10 years		75,000,000		75,000,000
Amortized cost - maturity in excess of 10 years		65,000,000		65,000,000
Amortized cost total		232,000,000		232,000,000
Fair value - maturity less than 1 year		10,000,000		10,000,000
Fair value - maturity between 1 and 5 years		84,000,000		84,000,000
Fair value - maturity between 6 and 10 years		76,000,000		76,000,000
Fair value - maturity in excess of 10 years		68,000,000		68,000,000
Fair value total		238,000,000		238,000,000
Available-for-sale Securities, Gross Realized Gain (Loss) [Abstract]
Proceeds from sales of NDT securities		62,000,000	$ 38,000,000	100,000,000	$ 65,000,000
Other proceeds from sales		0	0	0	3,000,000
Gross realized gains	[1]	8,000,000	5,000,000	13,000,000	8,000,000
Gross realized losses	[1]	6,000,000	$ 3,000,000	9,000,000	$ 4,000,000
NDT Funds
Available-for-sale Securities, Balance Sheet, Reported Amounts [Abstract]
Amortized cost		532,000,000		532,000,000		$ 522,000,000
Gross unrealized gains		427,000,000		427,000,000		428,000,000
Gross unrealized losses		1,000,000		1,000,000		0
Fair value		958,000,000		958,000,000		950,000,000
NDT Funds | Cash and cash equivalents
Available-for-sale Securities, Balance Sheet, Reported Amounts [Abstract]
Amortized cost		15,000,000		15,000,000		19,000,000
Gross unrealized gains		0		0		0
Gross unrealized losses		0		0		0
Fair value		15,000,000		15,000,000		19,000,000
NDT Funds | Equity securities
Available-for-sale Securities, Balance Sheet, Reported Amounts [Abstract]
Amortized cost		292,000,000		292,000,000		283,000,000
Gross unrealized gains		420,000,000		420,000,000		417,000,000
Gross unrealized losses		0		0		0
Fair value		712,000,000		712,000,000		700,000,000
NDT Funds | Debt securities
Available-for-sale Securities, Balance Sheet, Reported Amounts [Abstract]
Amortized cost		225,000,000		225,000,000		218,000,000
Gross unrealized gains		7,000,000		7,000,000		11,000,000
Gross unrealized losses		1,000,000		1,000,000		0
Fair value		231,000,000		231,000,000		229,000,000
NDT Funds | Receivables/payables, net
Available-for-sale Securities, Balance Sheet, Reported Amounts [Abstract]
Amortized cost		0		0		2,000,000
Gross unrealized gains		0		0		0
Gross unrealized losses		0		0		0
Fair value		0		0		2,000,000
Auction Rate Securities
Available-for-sale Securities, Balance Sheet, Reported Amounts [Abstract]
Amortized cost		7,000,000		7,000,000		8,000,000
Gross unrealized gains		0		0		0
Gross unrealized losses		0		0		0
Fair value		$ 7,000,000		$ 7,000,000		$ 8,000,000

[1] Excludes the impact of other-than-temporary impairment charges recognized on the Statements of Income.