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Pension and Other Postretirement Benefit Plans - Contributions (Details) - USD ($) $ in Millions |  | 12 Months Ended
--- | --- | ---
Jan. 15, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Employer Contributions to Defined Benefit Pension Plans - Common Stock Value |  | $ 0.0 | $ 13.5 | $ 0.0
ESOP Compensation Expense |  | $ 11.4 | $ 11.0 | $ 9.2
Common Stock [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Employer Contributions to Defined Benefit Pension Plans - Common Stock Shares |  | 0 | 216,000 | 0
Employer Contributions to Defined Benefit Pension Plans - Common Stock Value |  |  | $ 13.5 |
Pension Plan [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Employer Contributions to Defined Benefit Pension Plans - Cash |  | $ 15.0 | 1.7 | $ 6.3
Employer Contributions to Defined Benefit Pension Plans - Common Stock Value |  | $ 0.0 | 13.5 | 0.0
Defined Benefit Plan - Amendments |  | The non-union defined benefit pension plan was frozen in 2018, and does not allow further crediting of service or earnings to the plan. Further, it is closed to new participants. The Minnesota Power union defined benefit pension plan is also closed to new participants. |  |
Expected Employer Contributions in 2019 |  | $ 0.0 |  |
Pension Plan [Member] | Subsequent Event [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Employer Contributions to Defined Benefit Pension Plans - Cash | $ 10.4 |  |  |
Postretirement Health and Life [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Expected Employer Contributions in 2019 |  | 0.0 |  |
Postretirement Health and Life [Member] | VEBA [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Employer Contributions to Other Postretirement Benefit Plans |  | 0.0 | 0.0 | 0.0
Postretirement Health and Life [Member] | Irrevocable Grantor Trust [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Employer Contributions to Other Postretirement Benefit Plans |  | $ 0.0 | $ 0.0 | $ 0.0

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- Definition Amount of contributions expected to be received by defined benefit plan from employer in next fiscal year following latest fiscal year. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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