Commitments, Guarantees and Contingencies - Other Matters (Details) - USD ($)
$ in Millions |
12 Months Ended |
|
Dec. 31, 2019 |
Dec. 31, 2018 |
| BNI Energy Reclamation Liability [Member] |
|
|
| Guarantor Obligations [Line Items] |
|
|
| Estimated Obligation |
$ 67.3
|
|
| BNI Energy Reclamation Liability [Member] | Letters of Credit [Member] |
|
|
| Guarantor Obligations [Line Items] |
|
|
| Collateral |
64.3
|
|
| BNI Energy Reclamation Liability [Member] | Surety Bonds [Member] |
|
|
| Guarantor Obligations [Line Items] |
|
|
| Collateral |
67.7
|
|
| ALLETE Properties Development and Maintenance Obligations [Member] |
|
|
| Guarantor Obligations [Line Items] |
|
|
| Estimated Obligation |
2.3
|
|
| ALLETE Properties Development and Maintenance Obligations [Member] | Surety Bonds and Letters of Credit [Member] |
|
|
| Guarantor Obligations [Line Items] |
|
|
| Collateral |
4.8
|
|
| Town Center District Capital Improvement Bonds [Member] |
|
|
| Guarantor Obligations [Line Items] |
|
|
| Bonds |
$ 26.4
|
|
| Bond Interest Rate |
6.00%
|
|
| Bond Term (Years) |
31 years
|
|
| Ownership Percentage of Benefited Property |
53.00%
|
68.00%
|
| Annual Assessment |
$ 1.4
|
|
| Palm Coast Park District Special Assessment Bonds [Member] |
|
|
| Guarantor Obligations [Line Items] |
|
|
| Bonds |
$ 31.8
|
|
| Bond Interest Rate |
5.70%
|
|
| Bond Term (Years) |
31 years
|
|
| Ownership Percentage of Benefited Property |
0.00%
|
19.00%
|
| Superior, WI [Member] | Manufactured Gas Plant [Member] | SWL&P [Member] |
|
|
| Guarantor Obligations [Line Items] |
|
|
| Expected Period to Incur Remediation Costs |
four years
|
|