Employee Benefits (Tables)

3 Months Ended

Mar. 31, 2012

Compensation and Retirement Disclosure [Abstract]

Schedule of Defined Benefit Plans Disclosures

Postretirement benefit expense components are listed in the table below:

	3 Months Ended						12 Months Ended
	March 31,						March 31,
	2012			2011			2012			2011
	(in millions)
Service cost	$	0.1		$	0.2		$	0.4		$	0.6
Interest cost	0.9			1.1			4.0			4.3
Expected return on plan assets	(0.5		)	(0.7		)	(2.4		)	(2.6		)
Amortization of transition obligation	0.4			0.5			1.8			1.9
Amortization of losses	0.6			0.1			0.8			0.5
Curtailment costs	—			—			—			0.3
Accretion of regulatory liability	0.2			0.2			0.8			0.8
Postretirement benefits expense	$	1.7		$	1.4		$	5.4		$	5.8

	3 Months Ended						12 Months Ended
	March 31,						March 31,
	2012			2011			2012			2011
	(in millions)
Service cost	$	3.3		$	2.2		$	10.4		$	9.1
Interest cost	7.7			7.1			29.1			27.9
Expected return on plan assets	(7.7		)	(6.3		)	(27.4		)	(24.3		)
Prior service and other costs	0.2			0.3			1.0			1.1
Recognized net actuarial loss	5.4			2.6			14.9			9.0
Curtailment costs	0.1			—			0.9			2.2
Pension expense	$	9.0		$	5.9		$	28.9		$	25.0