Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2014

Fair Value Disclosures [Abstract]

Financial assets and liabilities by level within the fair value hierarchy

The following tables summarize our financial assets and liabilities by level within the fair value hierarchy:

Recurring Fair Value Measures	As of December 31, 2014
	Level 1		Level 2		Level 3		Total
	(Millions of Dollars)
Assets:
Derivatives	$	1.1	$	7.2	$	7.0	15.3
Total	$	1.1	$	7.2	$	7.0	$	15.3
Liabilities:
Derivatives	$	11.5	$	1.0	$	—	$	12.5
Total	$	11.5	$	1.0	$	—	$	12.5

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

The following table summarizes the changes to 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	15.6			10.6
Issuances	—			—
Settlements	(12.1		)	(11.8		)
Transfers in and/or out of Level 3	—			—
Balance as of December 31	$	7.0		$	3.5

Carrying amount and estimated fair value of certain financial instruments

The carrying amount and estimated fair value of certain of our recorded financial instruments as of December 31 are as follows:

	2014				2013
	Carrying		Fair		Carrying		Fair
Financial Instruments	Amount		Value		Amount		Value
	(Millions of Dollars)
Preferred stock, no redemption required	$	30.4	$	27.1	$	30.4	$	26.0
Long-term debt including current portion	$	4,552.4	$	5,126.0	$	4,626.7	$	4,911.8