Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2012

Fair Value Measurements

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

	September 30, 2012									December 31, 2011
	Adjusted Cost		Unrealized Gains		Unrealized Losses			Fair Value		Adjusted Cost		Unrealized Gains		Unrealized Losses			Fair Value
	(in millions)
Cash	$	41.4	$	—	$	—		$	41.4	$	99.8	$	—	$	—		$	99.8
U.S. and Canadian government obligations		2,172.4		—		—			2,172.4		515.0		—		—			515.0
Other debt securities		7.5		—		—			7.5		592.2		—		—			592.2
Total cash and cash equivalents	$	2,221.3	$	—	$	—		$	2,221.3	$	1,207.0	$	—	$	—		$	1,207.0
Investments in auction rate securities		44.6		—		(2.9	)		41.7		75.6		—		(4.7	)		70.9
Asset retirement obligation funds		147.6		—		—			147.6		145.4		—		—			145.4
Nonqualified employee benefit trusts		21.3		0.6		—			21.9		20.3		—		(0.1	)		20.2

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

	September 30, 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	$	2,221.3	$	2,221.3	$	—	$	—
Unrealized gains on natural gas derivatives		0.9		—		0.9		—
Asset retirement obligation funds		147.6		147.6		—		—
Investments in auction rate securities		41.7		—		—		41.7
Nonqualified employee benefit trusts		21.9		21.9		—		—
Total assets at fair value	$	2,433.4	$	2,390.8	$	0.9	$	41.7
Unrealized losses on natural gas derivatives	$	13.6	$	—	$	13.6	$	—
Total liabilities at fair value	$	13.6	$	—	$	13.6	$	—

	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 reconciliation of changes in Level 3 assets

	Investments in auction rate securities
	(in millions)
Fair value, December 31, 2011	$	70.9
Sales and redemptions		(31.0	)
Unrealized loss included in other comprehensive loss		1.8
Fair value, September 30, 2012	$	41.7