Derivative Instruments (Tables)

6 Months Ended

Jun. 30, 2015

Derivative Instruments (Tables) [Abstract]

Derivative instruments in balance sheet

	June 30, 2015				December 31, 2014
	Derivative Asset		Derivative Liability		Derivative Asset		Derivative Liability
	(Millions of Dollars)
Natural Gas	$	6.2	$	26.9	$	5.0	$	12.3
Fuel Oil	0.1		0.9		—		—
FTRs	7.7		—		7.0		—
Coal	1.3		10.9		3.3		0.2
Total	$	15.3	$	38.7	$	15.3	$	12.5

Derivative instruments' estimated notional volumes and gain (losses)

	Three Months Ended June 30, 2015				Three Months Ended June 30, 2014
	Volume	Gains (Losses)			Volume	Gains (Losses)
		(Millions of Dollars)				(Millions of Dollars)
Natural Gas	10.0 million Dth	$	(5.9	)	9.9 million Dth	$	2.5
Fuel Oil	0.8 million gallons	0.1			2.4 million gallons	0.4
FTRs	5.9 million MWh	0.8			7.4 million MWh	2.6
Total		$	(5.0	)		$	5.5

	Six Months Ended June 30, 2015				Six Months Ended June 30, 2014
	Volume	Gains (Losses)			Volume	Gains (Losses)
		(Millions of Dollars)				(Millions of Dollars)
Natural Gas	23.3 million Dth	$	(13.0	)	24.8 million Dth	$	10.1
Fuel Oil	1.7 million gallons	—			4.4 million gallons	0.6
FTRs	12.1 million MWh	2.9			13.1 million MWh	9.6
Total		$	(10.1	)		$	20.3

Offsetting Assets and Liabilities [Table Text Block]

	June 30, 2015						December 31, 2014
	Derivative			Derivative			Derivative			Derivative
	Asset			Liability			Asset			Liability
	(Millions of Dollars)
Gross Amount Recognized on the Balance Sheet	$	15.3		$	38.7		$	15.3		$	12.5
Gross Amount Not Offset on Balance Sheet (a)	(4.9		)	(8.9		)	(0.4		)	(11.5		)
Net Amount	$	10.4		$	29.8		$	14.9		$	1.0

(a) Gross Amount Not Offset on Balance Sheet includes cash collateral posted of $3.9 million and $10.3 million as of June 30, 2015 and December 31, 2014, respectively.