Employee Benefits (Tables)

3 Months Ended

Mar. 31, 2013

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						12 Months Ended
	March 31,						March 31,
	2013			2012			2013			2012
	(in millions)
Service cost	$	3.4		$	3.3		$	13.3		$	10.4
Interest cost	7.9			7.7			31.3			29.1
Expected return on plan assets	(9.5		)	(7.7		)	(33.6		)	(27.4		)
Prior-service and other costs	0.3			0.3			1.1			1.9
Recognized net actuarial loss	6.8			5.4			26.3			14.9
Pension expense	$	8.9		$	9.0		$	38.4		$	28.9

Postretirement benefit expense components are listed in the table below:

	3 Months Ended						12 Months Ended
	March 31,						March 31,
	2013			2012			2013			2012
	(in millions)
Service cost	$	0.2		$	0.1		$	0.8		$	0.4
Interest cost	1.0			0.9			3.9			4.0
Expected return on plan assets	(0.7		)	(0.5		)	(2.5		)	(2.4		)
Amortization of transition obligation	—			0.4			1.2			1.8
Amortization of losses	0.7			0.6			2.6			0.8
Accretion of regulatory liability	0.2			0.2			0.8			0.8
Postretirement benefits expense	$	1.4		$	1.7		$	6.8		$	5.4