Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2014

Fair Value Measurements (Tables) [Abstract]

Financial assets and liabilities by level within the fair value hierarchy

Recurring Fair Value Measures	As of June 30, 2014
	Level 1		Level 2		Level 3		Total
	(Millions of Dollars)
Assets:
Derivatives	$	2.2	$	6.6	$	14.1	$	22.9
Total	$	2.2	$	6.6	$	14.1	$	22.9
Liabilities:
Derivatives	$	—	$	0.2	$	—	$	0.2
Total	$	—	$	0.2	$	—	$	0.2

Recurring Fair Value Measures	As of December 31, 2013
	Level 1		Level 2		Level 3		Total
	(Millions of Dollars)
Assets:
Derivatives	$	5.7	$	2.6	$	3.5	$	11.8
Total	$	5.7	$	2.6	$	3.5	$	11.8
Liabilities:
Derivatives	$	—	$	0.3	$	—	$	0.3
Total	$	—	$	0.3	$	—	$	0.3

Fair value of derivatives classified as Level 3 in the fair value hierarchy

	Three Months Ended June 30						Six Months Ended June 30
	2014			2013			2014			2013
	(Millions of Dollars)
Beginning Balance	$	1.7		$	1.6		$	3.5		$	4.7
Realized and unrealized gains (losses)	—			—			—			—
Purchases	15.6			10.6			15.6			10.6
Issuances	—			—			—			—
Settlements	(3.2		)	(3.0		)	(5.0		)	(6.1		)
Transfers in and/or out of Level 3	—			—			—			—
Balance as of June 30	$	14.1		$	9.2		$	14.1		$	9.2
Change in unrealized gains (losses) relating to instruments still held as of June 30	$	—		$	—		$	—		$	—

Carrying amount and estimated fair value of certain financial instruments

	June 30, 2014				December 31, 2013
Financial Instruments	Carrying Amount		Fair Value		Carrying Amount		Fair Value
	(Millions of Dollars)
Preferred stock, no redemption required	$	30.4	$	25.8	$	30.4	$	26.0
Long-term debt, including current portion	$	4,565.6	$	4,995.4	$	4,626.7	$	4,911.8