Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2012

Fair Value Measurements

Schedule of cash and cash equivalents, short-term investments and other investments reconciliation from adjusted cost to fair value

	March 31, 2012									December 31, 2011
	Adjusted Cost		Unrealized Gains		Unrealized Losses			Fair Value		Adjusted Cost		Unrealized Gains		Unrealized Losses			Fair Value
	(in millions)
Cash	$	115.7	$	—	$	—		$	115.7	$	99.8	$	—	$	—		$	99.8
U.S. federal government obligations		1,545.8		—		—			1,545.8		515.0		—		—			515.0
Other debt securities		51.8		—		—			51.8		592.2		—		—			592.2
Total cash and cash equivalents	$	1,713.3	$	—	$	—		$	1,713.3	$	1,207.0	$	—	$	—		$	1,207.0
Investments in auction rate securities		75.6		—		(4.5	)		71.1		75.6		—		(4.7	)		70.9
Asset retirement obligation funds		147.6		—		—			147.6		145.4		—		—			145.4
Nonqualified employee benefit trusts		20.9		0.8		—			21.7		20.3		—		(0.1	)		20.2

Schedule of assets and liabilities measured at fair value on a recurring basis

	March 31, 2012
	Total Fair Value		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
	(in millions)
Cash and cash equivalents	$	1,713.3	$	1,713.3	$	—	$	—
Unrealized gains on natural gas derivatives		0.2		—		0.2		—
Asset retirement obligation funds		147.6		147.6		—		—
Investments in auction rate securities		71.1		—		—		71.1
Nonqualified employee benefit trusts		21.7		21.7		—		—
Total assets at fair value	$	1,953.9	$	1,882.6	$	0.2	$	71.1
Unrealized losses on natural gas derivatives	$	130.3	$	—	$	130.3	$	—
Total liabilities at fair value	$	130.3	$	—	$	130.3	$	—

	December 31, 2011
	Total Fair Value		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
	(in millions)
Cash and cash equivalents	$	1,207.0	$	1,207.0	$	—	$	—
Unrealized gains on natural gas derivatives		0.5		—		0.5		—
Asset retirement obligation funds		145.4		145.4		—		—
Investments in auction rate securities		70.9		—		—		70.9
Nonqualified employee benefit trusts		20.2		20.2		—		—
Total assets at fair value	$	1,444.0	$	1,372.6	$	0.5	$	70.9
Unrealized losses on natural gas derivatives	$	74.7	$	—	$	74.7	$	—
Total liabilities at fair value	$	74.7	$	—	$	74.7	$	—

Schedule of carrying values ARS by range of maturity

	(in millions)
Less than 1 year	$	—
1 year up to 20 years		4.8
20 years up to 30 years		51.8
30 years up to 36 years		14.5
	$	71.1

Schedule of reconciliation of assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3)

	Investments in auction rate securities
	(in millions)
Fair value, January 1, 2012	$	70.9
Unrealized gain included in other comprehensive loss		0.2
Fair value, March 31, 2012	$	71.1