Employee Benefits (Tables)

9 Months Ended

Sep. 30, 2012

Compensation and Retirement Disclosure [Abstract]

Schedule of Defined Benefit Plans Disclosures

Components of the qualified and nonqualified pension expense included in the determination of net income are listed in the table below:

	3 Months Ended						9 Months Ended						12 Months Ended
	September 30,						September 30,						September 30,
	2012			2011			2012			2011			2012			2011
	(in millions)
Service cost	$	3.2		$	2.2		$	9.7		$	6.6		$	12.4		$	8.7
Interest cost	7.8			7.1			23.3			21.3			30.5			27.7
Expected return on plan assets	(8.0		)	(6.3		)	(23.7		)	(18.9		)	(30.8		)	(24.9		)
Prior service and other costs	0.3			0.3			0.8			0.9			1.0			1.1
Recognized net actuarial loss	5.4			2.6			16.2			7.8			20.5			9.7
Curtailment costs	0.1			—			0.3			—			1.1			1.0
Pension expense	$	8.8		$	5.9		$	26.6		$	17.7		$	34.7		$	23.3

Postretirement benefit expense components are listed in the table below:

	3 Months Ended						9 Months Ended						12 Months Ended
	September 30,						September 30,						September 30,
	2012			2011			2012			2011			2012			2011
	(in millions)
Service cost	$	0.2		$	0.2		$	0.5		$	0.5		$	0.5		$	0.6
Interest cost	1.0			1.1			2.8			3.3			3.7			4.3
Expected return on plan assets	(0.6		)	(0.7		)	(1.7		)	(2.0		)	(2.3		)	(2.6		)
Amortization of transition obligation	0.4			0.4			1.2			1.3			1.8			1.8
Amortization of losses	0.5			0.2			1.6			0.5			1.4			0.6
Accretion of regulatory liability	0.2			0.2			0.6			0.6			0.8			0.8
Curtailment costs	—			—			—			—			—			0.3
Postretirement benefits expense	$	1.7		$	1.4		$	5.0		$	4.2		$	5.9		$	5.8