Employee Benefits (Tables)

6 Months Ended

Jun. 30, 2015

Compensation and Retirement Disclosure [Abstract]

Schedule of Defined Benefit Plans Disclosures

Components of the qualified and nonqualified net periodic pension cost are listed in the table below:

	3 Months Ended						6 Months Ended						12 Months Ended
	June 30,						June 30,						June 30,
	2015			2014			2015			2014			2015			2014
	(in millions)
Service cost	$	3.4		$	3.0		$	6.8		$	6.0		$	12.6		$	12.8
Interest cost	8.1			8.3			16.2			16.7			32.9			31.8
Expected return on plan assets	(11.7		)	(10.7		)	(23.3		)	(21.3		)	(45.6		)	(40.3		)
Prior service cost	—			0.2			—			0.3			0.3			0.8
Recognized net actuarial loss	5.7			4.0			11.4			7.9			18.9			23.1
Net pension cost	$	5.5		$	4.8		$	11.1		$	9.6		$	19.1		$	28.2

Net periodic 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,
	2015			2014			2015			2014			2015			2014
	(in millions)
Service cost	$	0.1		$	0.1		$	0.3		$	0.3		$	0.6		$	0.6
Interest cost	0.9			1.0			1.8			2.0			3.6			3.8
Expected return on plan assets	(0.8		)	(0.7		)	(1.6		)	(1.5		)	(3.1		)	(2.8		)
Recognized net actuarial loss	0.3			0.3			0.6			0.6			0.7			2.2
Accretion of regulatory liability	0.2			0.2			0.4			0.4			0.8			0.6
Net postretirement benefit cost	$	0.7		$	0.9		$	1.5		$	1.8		$	2.6		$	4.4