Other Investments (Tables)

3 Months Ended

Mar. 31, 2013

Regulated Entity, Other Assets, Noncurrent [Abstract]

Amortized Cost and Fair Value of the Trust Investments

The following table sets forth amortized cost and fair value of the trust investments:

	Longest Maturity Dates	Amortized Cost				Fair Value
(in millions)		March 31, 2013		December 31, 2012		March 31, 2013		December 31, 2012
Stocks	—	$	990	$	978	$	2,468	$	2,271
Municipal bonds	2051	509		518		631		644
U.S. government and agency securities	2043	590		547		646		603
Corporate bonds	2054	327		324		408		410
Short-term investments and receivables/payables	One-year	89		116		93		120
Total		$	2,505	$	2,483	$	4,246	$	4,048

Summary of Changes in the Fair Value of Trust

The following table sets forth a summary of changes in the fair value of the trust:

	Three months ended March 31,
(in millions)	2013			2012
Balance at beginning of period	$	4,048		$	3,592
Gross realized gains	5			25
Gross realized losses	(1		)	(4		)
Unrealized gains, net	176			184
Other-than-temporary impairments	(8		)	(5		)
Interest, dividends, contributions and other	26			61
Balance at end of period	$	4,246		$	3,853