Income Taxes (Tables)

12 Months Ended

Dec. 31, 2016

Income Tax Disclosure [Abstract]

Reconciliation of Unrecognized Tax Benefits

A reconciliation of the amount of unrecognized tax benefit is as follows:

(Millions of Dollars)	Dec. 31, 2016		Dec. 31, 2015
Unrecognized tax benefit — Permanent tax positions	$	2.9	$	2.4
Unrecognized tax benefit — Temporary tax positions	16.8		15.0
Total unrecognized tax benefit	$	19.7	$	17.4

A reconciliation of the beginning and ending amount of unrecognized tax benefit is as follows:

(Millions of Dollars)	2016			2015			2014
Balance at Jan. 1	$	17.4		$	11.9		$	8.4
Additions based on tax positions related to the current year	2.7			4.5			3.7
Reductions based on tax positions related to the current year	—			(1.5		)	(0.7		)
Additions for tax positions of prior years	0.5			2.5			2.8
Reductions for tax positions of prior years	(0.9		)	—			(1.2		)
Settlements with taxing authorities	—			—			(1.1		)
Balance at Dec. 31	$	19.7		$	17.4		$	11.9

Tax Benefits Associated with NOL and Tax Credit Carryforwards

The unrecognized tax benefit amounts were reduced by the tax benefits associated with NOL and tax credit carryforwards.  The amounts of tax benefits associated with NOL and tax credit carryforwards are as follows:

(Millions of Dollars)	Dec. 31, 2016			Dec. 31, 2015
NOL and tax credit carryforwards	$	(5.8	)	$	(4.3	)

Interest Payable related to Unrecognized Tax Benefits [Table Text Block]

The payable for interest related to unrecognized tax benefits is partially offset by the interest benefit associated with NOL and tax credit carryforwards. A reconciliation of the beginning and ending amount of the payable for interest related to unrecognized tax benefits are as follows:

(Millions of Dollars)	2016			2015			2014
Payable for interest related to unrecognized tax benefits at Jan. 1	$	(0.4	)	$	(0.2	)	$	(0.1	)
Interest expense related to unrecognized tax benefits	(0.7		)	(0.2		)	(0.1		)
Payable for interest related to unrecognized tax benefits at Dec. 31	$	(1.1	)	$	(0.4	)	$	(0.2	)

NOL and Tax Credit Carryforwards

Other Income Tax Matters — NOL amounts represent the amount of the tax loss that is carried forward and tax credits represent the deferred tax asset. NOL and tax credit carryforwards as of Dec. 31 were as follows:

(Millions of Dollars)	2016			2015
Federal NOL carryforward	$	260		$	328
Federal tax credit carryforwards	25			24
State NOL carryforwards	684			684
State tax credit carryforwards, net of federal detriment (a)	13			13
Valuation allowances for state credit carryforwards, net of federal detriment (b)	(3		)	(1		)

^(a)  State tax credit carryforwards are net of federal detriment of $7 million and $7 million as of Dec. 31, 2016 and 2015, respectively.

^(b)  Valuation allowances for state tax credit carryforwards were net of federal benefit of $2 million and $1 million as of Dec. 31, 2016 and 2015, respectively.

Schedule of Effective Income Tax Rate Reconciliation

Total income tax expense from operations differs from the amount computed by applying the statutory federal income tax rate to income before income tax expense.  The following reconciles such differences for the years ending Dec. 31:

	2016		2015		2014
Federal statutory rate	35.0	%	35.0	%	35.0	%
Increases (decreases) in tax from:
State income taxes, net of federal income tax benefit	3.2		3.2		2.8
Tax credits recognized, net of federal income tax expense	(0.7	)	(0.7	)	(0.8	)
Regulatory differences — utility plant items	(0.5	)	(0.3	)	(2.1	)
Change in unrecognized tax benefits (expense)	—		0.1		(0.1	)
Other, net	0.1		0.1		0.1
Effective income tax rate	37.1	%	37.4	%	34.9	%

Schedule of Components of Income Tax Expense (Benefit)

The components of income tax expense for the years ending Dec. 31 were:

(Thousands of Dollars)	2016			2015			2014
Current federal tax expense (benefit)	$	45,287		$	(1,166	)	$	9,550
Current state tax expense (benefit)	8,754			(727		)	2,611
Current change in unrecognized tax expense	680			5,244			6,548
Deferred federal tax expense	195,064			246,096			208,781
Deferred state tax expense	27,216			36,450			26,196
Deferred change in unrecognized tax benefit	(278		)	(4,650		)	(7,154		)
Deferred investment tax credits	(2,805		)	(2,807		)	(2,941		)
Total income tax expense	$	273,918		$	278,440		$	243,591

The components of deferred income tax expense for the years ending Dec. 31 were:

(Thousands of Dollars)	2016			2015			2014
Deferred tax expense excluding items below	$	230,931		$	285,144		$	254,142
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities	(8,418		)	(7,229		)	(26,649		)
Tax (expense) benefit allocated to other comprehensive income and other	(511		)	(19		)	330
Deferred tax expense	$	222,002		$	277,896		$	227,823

Schedule of Deferred Tax Assets and Liabilities

The components of the net deferred tax liability at Dec. 31 were as follows:

(Thousands of Dollars)	2016		2015
Deferred tax liabilities:
Differences between book and tax bases of property	$	2,967,162	$	2,772,043
Employee benefits	87,693		105,049
Other	69,931		82,732
Total deferred tax liabilities	$	3,124,786	$	2,959,824
Deferred tax assets:
NOL carryforward	$	115,328	$	147,763
Tax credit carryforward	34,658		35,240
Regulatory liabilities	19,635		17,201
Deferred investment tax credits	11,653		12,718
Deferred fuel costs	10,070		29,694
Rate refund	7,221		23,352
Other	37,092		35,658
Total deferred tax assets	$	235,657	$	301,626
Net deferred tax liability	$	2,889,129	$	2,658,198