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Income Taxes (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Income tax provision components [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Current payable: Federal |  |  |  |  |  |  |  |  | $ 16,837 | $ (891) | $ (1,508) |  |  |
Current payable: State |  |  |  |  |  |  |  |  | 2,774 | (589) | 8,213 |  |  |
Net-deferred: Federal |  |  |  |  |  |  |  |  | 15,951 | 39,284 | 37,203 |  |  |
Net-deferred: State |  |  |  |  |  |  |  |  | 5,976 | 10,600 | 1,163 |  |  |
Amortized investment tax credits |  |  |  |  |  |  |  |  | (175) | (219) | (212) |  |  |
Total income tax provision | $ 7,417 | $ 15,351 | $ 8,008 | $ 10,587 | $ 8,600 | $ 14,286 | $ 9,034 | $ 16,265 | $ 41,363 | $ 48,185 | $ 44,859 |  |  |
Reconciliation of tax provision to statutory federal income tax rate [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Statutory federal income tax rate |  |  |  |  |  |  |  |  | 35.00% | 35.00% | 35.00% |  |  |
State income taxes, net of federal benefit |  |  |  |  |  |  |  |  | 5.20% | 5.10% | 5.10% |  |  |
Amortized investment tax credits |  |  |  |  |  |  |  |  | (0.20%) | (0.20%) | (0.20%) |  |  |
Credit for electricity from wind energy |  |  |  |  |  |  |  |  | (1.80%) | (1.70%) | (1.50%) |  |  |
Domestic manufacturing deduction |  |  |  |  |  |  |  |  | (1.40%) | 0.00% | (0.20%) |  |  |
AFUDC Equity, net |  |  |  |  |  |  |  |  | (0.10%) | (0.80%) | (0.70%) |  |  |
Other, net, individually significant |  |  |  |  |  |  |  |  | 0.00% | 0.10% | 0.00% |  |  |
Effective income tax rate |  |  |  |  |  |  |  |  | 36.70% | 37.50% | 37.50% |  |  |
Deferred tax liabilities (assets) on balance sheet [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property-related |  |  |  |  |  |  |  |  |  |  |  | $ 327,918 | $ 312,903 |
Investment in ATC |  |  |  |  |  |  |  |  |  |  |  | 38,213 | 36,140 |
Bond transactions |  |  |  |  |  |  |  |  |  |  |  | 1,422 | 1,420 |
Pension and other postretirement benefits |  |  |  |  |  |  |  |  |  |  |  | 57,697 | 57,847 |
Derivatives |  |  |  |  |  |  |  |  |  |  |  | 21,660 | 22,331 |
Tax deductible prepayments |  |  |  |  |  |  |  |  |  |  |  | 8,011 | 8,077 |
Other deferred tax liabilities |  |  |  |  |  |  |  |  |  |  |  | 14,997 | 10,451 |
Gross deferred income tax liabilities |  |  |  |  |  |  |  |  |  |  |  | 469,918 | 449,169 |
Future tax benefit |  |  |  |  |  |  |  |  |  |  |  | 0 | (4,092) |
Accrued expenses |  |  |  |  |  |  |  |  |  |  |  | (21,391) | (32,091) |
Pension and other postretirement benefits |  |  |  |  |  |  |  |  |  |  |  | (46,582) | (44,994) |
Deferred tax regulatory account |  |  |  |  |  |  |  |  |  |  |  | (1,047) | (1,211) |
Derivatives |  |  |  |  |  |  |  |  |  |  |  | (21,660) | (22,331) |
Other deferred tax assets |  |  |  |  |  |  |  |  |  |  |  | (18,523) | (5,957) |
Gross deferred income tax assets |  |  |  |  |  |  |  |  |  |  |  | (109,203) | (110,676) |
Less valuation allowance |  |  |  |  |  |  |  |  |  |  |  | 70 | 70 |
Net deferred income tax assets |  |  |  |  |  |  |  |  |  |  |  | (109,133) | (110,606) |
Deferred income taxes |  |  |  |  |  |  |  |  |  |  |  | 360,785 | 338,563 |
Operating loss deductions and tax credit carryforwards [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
State net tax operating loss deductions |  |  |  |  |  |  |  |  |  |  |  | 1,400 |  |
Unrecognized tax benefits [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unrecognized tax benefits, beginning balance |  |  |  | 2,365 |  |  |  | 2,363 | $ 2,365 | $ 2,363 | $ 3,204 |  |  |
Additions based on tax positions related to current year |  |  |  |  |  |  |  |  | 488 | 610 | 377 |  |  |
Additions based on tax positions related to prior year |  |  |  |  |  |  |  |  | 520 | 618 | 424 |  |  |
Reductions based on tax positions related to the current year |  |  |  |  |  |  |  |  | 0 | 0 | (40) |  |  |
Reductions based on tax positions related to prior years |  |  |  |  |  |  |  |  | (845) | (1,226) | (1,602) |  |  |
Unrecognized tax benefits, ending balance | 2,528 |  |  |  | 2,365 |  |  |  | 2,528 | 2,365 | 2,363 |  |  |
Interest on unrecognized tax benefits [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accrued interest on unrecognized tax benefits, beginning balance |  |  |  | 92 |  |  |  | 101 | 92 | 101 | 314 |  |  |
Reduction in interest expense on uncertain tax positions |  |  |  |  |  |  |  |  | (102) | (97) | (275) |  |  |
Interest expense on uncertain tax positions |  |  |  |  |  |  |  |  | 321 | 88 | 62 |  |  |
Accrued interest on unrecognized tax benefits, ending balance | 311 |  |  |  | 92 |  |  |  | 311 | 92 | 101 |  |  |
Unrecognized Tax Benefits |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unrecognized tax benefits | 2,528 |  |  | 2,365 | 2,365 |  |  | $ 2,363 | 2,365 | 2,365 | 2,363 | 2,528 | 2,365 | $ 2,363
Unrecognized tax benefits permanent differences |  |  |  |  |  |  |  |  |  |  |  | 0 | 0 | $ 0
Other deferred liabilities [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unrecognized tax benefits [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unrecognized tax benefits, beginning balance |  |  |  | 400 |  |  |  |  | 400 |  |  |  |  |
Unrecognized tax benefits, ending balance | 2,500 |  |  |  | 400 |  |  |  | 2,500 | 400 |  |  |  |
Unrecognized Tax Benefits |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unrecognized tax benefits | $ 2,500 |  |  | 400 | 400 |  |  |  | 2,500 | 400 |  | 2,500 | 400 |
Deferred tax assets [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unrecognized tax benefits [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unrecognized tax benefits, beginning balance |  |  |  | 2,000 |  |  |  |  | 2,000 |  |  |  |  |
Unrecognized tax benefits, ending balance |  |  |  |  | 2,000 |  |  |  |  | 2,000 |  |  |  |
Unrecognized Tax Benefits |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unrecognized tax benefits |  |  |  | 2,000 | 2,000 |  |  |  | 2,000 | 2,000 |  |  | 2,000 |
Current assets [Member] | Accounting Standards Update 2015-17 - Deferred Income Taxes [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncement Or Change In Accounting Principle Retrospective Adjustments [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
New accounting pronouncement, effect of adoption, change in assets (deferred income taxes) |  |  |  |  |  |  |  |  |  | (3,500) |  |  |  |
Deferred liabilities [Member] | Accounting Standards Update 2015-17 - Deferred Income Taxes [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncement Or Change In Accounting Principle Retrospective Adjustments [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
New accounting pronouncement, effect of adoption, change in liabilities (deferred income taxes) |  |  |  |  |  |  |  |  |  | (3,500) |  |  |  |
State [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Operating loss deductions and tax credit carryforwards [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Operating loss carryforward |  |  |  |  |  |  |  |  |  |  |  | 0 | 16,500 |
Operating loss carryforward, deferred tax asset |  |  |  |  |  |  |  |  |  |  |  | 0 | 800 |
Federal [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Operating loss deductions and tax credit carryforwards [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Tax credit carryforward, amount |  |  |  |  |  |  |  |  |  |  |  | 0 | 5,400 |
Tax credit carryforward, deferred tax asset |  |  |  |  |  |  |  |  |  |  |  | 0 | 5,400 |
MGE [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income tax provision components [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Current payable: Federal |  |  |  |  |  |  |  |  | 19,295 | 637 | (448) |  |  |
Current payable: State |  |  |  |  |  |  |  |  | 3,443 | (451) | 8,322 |  |  |
Net-deferred: Federal |  |  |  |  |  |  |  |  | 13,538 | 38,553 | 36,937 |  |  |
Net-deferred: State |  |  |  |  |  |  |  |  | 5,305 | 10,625 | 1,223 |  |  |
Amortized investment tax credits |  |  |  |  |  |  |  |  | (175) | (219) | (212) |  |  |
Total income tax provision |  |  |  |  |  |  |  |  | $ 41,406 | $ 49,145 | $ 45,822 |  |  |
Reconciliation of tax provision to statutory federal income tax rate [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Statutory federal income tax rate |  |  |  |  |  |  |  |  | 35.00% | 35.00% | 35.00% |  |  |
State income taxes, net of federal benefit |  |  |  |  |  |  |  |  | 5.20% | 5.10% | 5.10% |  |  |
Amortized investment tax credits |  |  |  |  |  |  |  |  | (0.20%) | (0.20%) | (0.20%) |  |  |
Credit for electricity from wind energy |  |  |  |  |  |  |  |  | (1.80%) | (1.70%) | (1.50%) |  |  |
Domestic manufacturing deduction |  |  |  |  |  |  |  |  | (1.40%) | 0.00% | (0.20%) |  |  |
AFUDC Equity, net |  |  |  |  |  |  |  |  | (0.10%) | (0.80%) | (0.70%) |  |  |
Other, net, individually significant |  |  |  |  |  |  |  |  | 0.00% | 0.10% | 0.00% |  |  |
Effective income tax rate |  |  |  |  |  |  |  |  | 36.70% | 37.50% | 37.50% |  |  |
Deferred tax liabilities (assets) on balance sheet [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property-related |  |  |  |  |  |  |  |  |  |  |  | 327,822 | 312,807 |
Investment in ATC |  |  |  |  |  |  |  |  |  |  |  | 30,382 | 29,156 |
Bond transactions |  |  |  |  |  |  |  |  |  |  |  | 1,422 | 1,420 |
Pension and other postretirement benefits |  |  |  |  |  |  |  |  |  |  |  | 57,697 | 57,847 |
Derivatives |  |  |  |  |  |  |  |  |  |  |  | 21,660 | 22,331 |
Tax deductible prepayments |  |  |  |  |  |  |  |  |  |  |  | 8,011 | 8,077 |
Other deferred tax liabilities |  |  |  |  |  |  |  |  |  |  |  | 14,831 | 10,259 |
Gross deferred income tax liabilities |  |  |  |  |  |  |  |  |  |  |  | 461,825 | 441,897 |
Future tax benefit |  |  |  |  |  |  |  |  |  |  |  | 0 | (4,092) |
Accrued expenses |  |  |  |  |  |  |  |  |  |  |  | (21,391) | (32,091) |
Pension and other postretirement benefits |  |  |  |  |  |  |  |  |  |  |  | (46,582) | (44,994) |
Deferred tax regulatory account |  |  |  |  |  |  |  |  |  |  |  | (1,047) | (1,211) |
Derivatives |  |  |  |  |  |  |  |  |  |  |  | (21,660) | (22,331) |
Other deferred tax assets |  |  |  |  |  |  |  |  |  |  |  | (18,589) | (3,746) |
Gross deferred income tax assets |  |  |  |  |  |  |  |  |  |  |  | (109,269) | (108,465) |
Less valuation allowance |  |  |  |  |  |  |  |  |  |  |  | 70 | 70 |
Net deferred income tax assets |  |  |  |  |  |  |  |  |  |  |  | (109,199) | (108,395) |
Deferred income taxes |  |  |  |  |  |  |  |  |  |  |  | 352,626 | 333,502 |
Operating loss deductions and tax credit carryforwards [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
State net tax operating loss deductions |  |  |  |  |  |  |  |  |  |  |  | 1,400 |  |
MGE [Member] | Deferred tax assets [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unrecognized tax benefits [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unrecognized tax benefits, beginning balance |  |  |  | 2,000 |  |  |  |  | $ 2,000 |  |  |  |  |
Unrecognized tax benefits, ending balance |  |  |  |  | 2,000 |  |  |  |  | $ 2,000 |  |  |  |
Unrecognized Tax Benefits |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unrecognized tax benefits |  |  |  | $ 2,000 | $ 2,000 |  |  |  | $ 2,000 | 2,000 |  |  | 2,000 |
MGE [Member] | Current assets [Member] | Accounting Standards Update 2015-17 - Deferred Income Taxes [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncement Or Change In Accounting Principle Retrospective Adjustments [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
New accounting pronouncement, effect of adoption, change in assets (deferred income taxes) |  |  |  |  |  |  |  |  |  | (1,300) |  |  |  |
MGE [Member] | Deferred liabilities [Member] | Accounting Standards Update 2015-17 - Deferred Income Taxes [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncement Or Change In Accounting Principle Retrospective Adjustments [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
New accounting pronouncement, effect of adoption, change in liabilities (deferred income taxes) |  |  |  |  |  |  |  |  |  | $ (1,300) |  |  |  |
MGE [Member] | State [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Operating loss deductions and tax credit carryforwards [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Operating loss carryforward |  |  |  |  |  |  |  |  |  |  |  | 0 | 16,500 |
Operating loss carryforward, deferred tax asset |  |  |  |  |  |  |  |  |  |  |  | 0 | 800 |
MGE [Member] | Federal [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Operating loss deductions and tax credit carryforwards [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Tax credit carryforward, amount |  |  |  |  |  |  |  |  |  |  |  | 0 | 5,400 |
Tax credit carryforward, deferred tax asset |  |  |  |  |  |  |  |  |  |  |  | $ 0 | $ 5,400 |

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