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Employee Benefit Plans Defined Benefit Plans Assumptions (Details) (USD $)
In Thousands, unless otherwise specified
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12 Months Ended |
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Dec. 31, 2013
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Dec. 31, 2012
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Dec. 31, 2011
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| Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract] |
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| Assumptions, Expected Long-term Return on Assets |
7.25% |
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7.25% |
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| Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates [Abstract] |
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| Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation |
$ 1,914 |
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| Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation |
(1,644) |
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| Effect of One Percentage Point Increase on Service and Interest Cost Components |
136 |
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| Effect of One Percentage Point Decrease on Service and Interest Cost Components |
$ (116) |
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Pension Plans, Defined Benefit [Member]
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| Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract] |
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| Assumptions, Discount Rate, Benefit Obligation |
5.05% |
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4.30% |
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4.65% |
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| Assumptions, Rate of Increase in Compensation Levels |
3.78% |
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3.84% |
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3.77% |
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| Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract] |
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| Assumptions, Expected Long-term Return on Assets |
7.25% |
[1] |
7.25% |
[1] |
7.75% |
[1] |
| Assumptions, Rate of Compensation Increase |
3.78% |
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3.75% |
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3.79% |
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| Defined Benefit Plan Assumptions Used In Calculating Net Periodic Benefit Cost Expected Rate of Return On Assets For Next Fiscal Year |
6.75% |
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Supplemental Employee Retirement Plans, Defined Benefit [Member]
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| Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract] |
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| Assumptions, Discount Rate, Benefit Obligation |
4.21% |
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3.44% |
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4.30% |
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| Assumptions, Rate of Increase in Compensation Levels |
5.00% |
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5.00% |
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5.00% |
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| Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract] |
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| Assumptions, Rate of Compensation Increase |
5.00% |
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5.00% |
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5.00% |
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Other Postretirement Benefit Plans, Defined Benefit [Member]
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| Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract] |
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| Assumptions, Discount Rate, Benefit Obligation |
4.62% |
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3.85% |
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4.42% |
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| Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract] |
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| Assumptions, Expected Long-term Return on Assets |
2.00% |
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2.00% |
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4.00% |
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Black Hills Corporation [Member] | Pension Plans, Defined Benefit [Member]
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| Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract] |
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| Assumptions, Discount Rate, Net Periodic Cost |
4.35% |
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4.68% |
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5.50% |
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Black Hills Corporation [Member] | Supplemental Employee Retirement Plans, Defined Benefit [Member]
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| Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract] |
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| Assumptions, Discount Rate, Net Periodic Cost |
3.88% |
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4.70% |
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5.00% |
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Black Hills Corporation [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
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| Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract] |
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| Assumptions, Discount Rate, Net Periodic Cost |
3.65% |
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4.35% |
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5.00% |
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Black Hills Utility Holdings [Member] | Pension Plans, Defined Benefit [Member]
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| Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract] |
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| Assumptions, Discount Rate, Net Periodic Cost |
4.25% |
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4.60% |
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5.40% |
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Black Hills Utility Holdings [Member] | Supplemental Employee Retirement Plans, Defined Benefit [Member]
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| Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract] |
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| Assumptions, Discount Rate, Net Periodic Cost |
3.00% |
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3.90% |
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4.40% |
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Black Hills Utility Holdings [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
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| Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract] |
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| Assumptions, Discount Rate, Net Periodic Cost |
3.50% |
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4.35% |
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4.60% |
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Cheyenne Light [Member] | Pension Plans, Defined Benefit [Member]
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| Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract] |
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| Assumptions, Discount Rate, Net Periodic Cost |
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5.55% |
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Cheyenne Light [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
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| Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract] |
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| Assumptions, Discount Rate, Net Periodic Cost |
4.40% |
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4.65% |
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5.50% |
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Age, Lower Than Retirement [Member] | Black Hills Corporation [Member]
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| Defined Benefit Plan, Assumed Health Care Cost Trend Rates [Abstract] |
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| Health Care Cost Trend Rate Assumed for Next Fiscal Year |
7.50% |
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7.75% |
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| Ultimate Health Care Cost Trend Rate |
4.50% |
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4.50% |
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| Year that Rate Reaches Ultimate Trend Rate |
2027 |
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2027 |
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Age, Lower Than Retirement [Member] | Black Hills Utility Holdings [Member]
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| Defined Benefit Plan, Assumed Health Care Cost Trend Rates [Abstract] |
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| Health Care Cost Trend Rate Assumed for Next Fiscal Year |
7.50% |
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7.75% |
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| Ultimate Health Care Cost Trend Rate |
4.50% |
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4.50% |
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| Year that Rate Reaches Ultimate Trend Rate |
2027 |
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2027 |
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Age, Lower Than Retirement [Member] | Cheyenne Light [Member]
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| Defined Benefit Plan, Assumed Health Care Cost Trend Rates [Abstract] |
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| Health Care Cost Trend Rate Assumed for Next Fiscal Year |
7.50% |
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7.75% |
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| Ultimate Health Care Cost Trend Rate |
4.50% |
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4.50% |
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| Year that Rate Reaches Ultimate Trend Rate |
2027 |
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2027 |
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Age, Greater Than Retirement [Member] | Black Hills Corporation [Member]
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| Defined Benefit Plan, Assumed Health Care Cost Trend Rates [Abstract] |
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| Health Care Cost Trend Rate Assumed for Next Fiscal Year |
6.25% |
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6.50% |
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| Ultimate Health Care Cost Trend Rate |
4.50% |
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4.50% |
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| Year that Rate Reaches Ultimate Trend Rate |
2026 |
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2026 |
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Age, Greater Than Retirement [Member] | Black Hills Utility Holdings [Member]
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| Defined Benefit Plan, Assumed Health Care Cost Trend Rates [Abstract] |
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| Health Care Cost Trend Rate Assumed for Next Fiscal Year |
6.25% |
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6.50% |
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| Ultimate Health Care Cost Trend Rate |
4.50% |
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4.50% |
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| Year that Rate Reaches Ultimate Trend Rate |
2026 |
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2026 |
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Age, Greater Than Retirement [Member] | Cheyenne Light [Member]
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| Defined Benefit Plan, Assumed Health Care Cost Trend Rates [Abstract] |
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| Health Care Cost Trend Rate Assumed for Next Fiscal Year |
6.25% |
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6.50% |
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| Ultimate Health Care Cost Trend Rate |
4.50% |
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4.50% |
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| Year that Rate Reaches Ultimate Trend Rate |
2026 |
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2026 |
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