Employee Benefits (Tables)

6 Months Ended

Jun. 30, 2013

Compensation and Retirement Disclosure [Abstract]

Schedule of Defined Benefit Plans Disclosures

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

	3 Months Ended						6 Months Ended						12 Months Ended
	June 30,						June 30,						June 30,
	2013			2012			2013			2012			2013			2012
	(in millions)
Service cost	$	3.4		$	3.2		$	6.8		$	6.5		$	13.5		$	11.4
Interest cost	7.9			7.8			15.8			15.5			31.4			29.8
Expected return on plan assets	(9.6		)	(8.0		)	(19.1		)	(15.7		)	(35.2		)	(29.1		)
Prior service and other costs	0.3			0.4			0.6			0.7			1.0			2.0
Recognized net actuarial loss	6.9			5.4			13.7			10.8			27.8			17.7
Net pension cost	$	8.9		$	8.8		$	17.8		$	17.8		$	38.5		$	31.8

Net postretirement benefit cost components are listed in the table below:

	3 Months Ended						6 Months Ended						12 Months Ended
	June 30,						June 30,						June 30,
	2013			2012			2013			2012			2013			2012
	(in millions)
Service cost	$	0.2		$	0.2		$	0.4		$	0.3		$	0.8		$	0.5
Interest cost	0.9			0.9			1.9			1.8			3.9			3.8
Expected return on plan assets	(0.6		)	(0.6		)	(1.3		)	(1.1		)	(2.5		)	(2.4		)
Amortization of transition obligation	—			0.4			—			0.8			0.8			1.8
Recognized net actuarial loss	0.7			0.5			1.4			1.1			2.8			1.1
Accretion of regulatory liability	0.2			0.2			0.4			0.4			0.8			0.8
Net postretirement benefit cost	$	1.4		$	1.6		$	2.8		$	3.3		$	6.6		$	5.6