Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2014

Fair Value Measurements (Tables) [Abstract]

Financial assets and liabilities by level within the fair value hierarchy

Recurring Fair Value Measures	As of March 31, 2014
	Level 1		Level 2		Level 3		Total
	(Millions of Dollars)
Assets:
Derivatives	$	3.6	$	3.9	$	1.7	$	9.2
Total	$	3.6	$	3.9	$	1.7	$	9.2
Liabilities:
Derivatives	$	—	$	0.1	$	—	$	0.1
Total	$	—	$	0.1	$	—	$	0.1

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

	2014			2013
	(Millions of Dollars)
Balance as of January 1	$	3.5		$	4.7
Realized and unrealized gains (losses)	—			—
Purchases	—			—
Issuances	—			—
Settlements	(1.8		)	(3.1		)
Transfers in and/or out of Level 3	—			—
Balance as of March 31	$	1.7		$	1.6
Change in unrealized gains (losses) relating to instruments still held as of March 31	$	—		$	—

Carrying amount and estimated fair value of certain financial instruments

	March 31, 2014				December 31, 2013
Financial Instruments	Carrying Amount		Fair Value		Carrying Amount		Fair Value
	(Millions of Dollars)
Preferred stock, no redemption required	$	30.4	$	25.4	$	30.4	$	26.0
Long-term debt, including current portion	$	4,617.8	$	4,954.2	$	4,626.7	$	4,911.8