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Retirement Plans and Postretirement Benefit Plans (Tables) | 6 Months Ended
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Jun. 30, 2018
Defined Benefit Plan [Abstract] |
Schedule of Defined Benefit Plans Disclosures [Table Text Block] | The following tables present the net periodic benefit cost components, before consideration of capitalized amounts, of the Company's Pension Plan, Restoration of Retirement Income Plan and postretirement benefit plans that are included in the Condensed Consolidated Financial Statements. Service cost is presented within Other Operation and Maintenance, and interest cost, expected return on plan assets, amortization of net loss and amortization of unrecognized prior service cost are presented within Other Net Periodic Benefit Income (Expense) on the Company's Condensed Consolidated Statements of Income.
| Pension Plan |  | Restoration of Retirement Income Plan
| Three Months Ended | Six Months Ended |  | Three Months Ended | Six Months Ended
| June 30, | June 30, |  | June 30, | June 30,
(In millions) | 2018 (A) | 2017 (A) | 2018 (B) | 2017 (B) |  | 2018 (A) | 2017 (A) | 2018 (B) | 2017 (B)
Service cost | $ | 3.4 |  | $ | 3.5 |  | $ | 7.5 |  | $ | 7.7 |  |  | $ | 0.1 |  | $ | 0.1 |  | $ | 0.2 |  | $ | 0.2 |
Interest cost | 5.8 |  | 6.6 |  | 11.7 |  | 13.1 |  |  | 0.1 |  | — |  | 0.2 |  | 0.1 |
Expected return on plan assets | (11.0 | ) | (10.6 | ) | (22.3 | ) | (21.3 | ) |  | — |  | — |  | — |  | — |
Amortization of net loss | 4.4 |  | 4.7 |  | 8.3 |  | 8.7 |  |  | 0.3 |  | 0.1 |  | 0.4 |  | 0.2 |
Total net periodic benefit cost | 2.6 |  | 4.2 |  | 5.2 |  | 8.2 |  |  | 0.5 |  | 0.2 |  | 0.8 |  | 0.5 |
Less: Amount paid by unconsolidated affiliates | 0.7 |  | 0.9 |  | 1.2 |  | 1.7 |  |  | — |  | — |  | — |  | — |
Net periodic benefit cost | $ | 1.9 |  | $ | 3.3 |  | $ | 4.0 |  | $ | 6.5 |  |  | $ | 0.5 |  | $ | 0.2 |  | $ | 0.8 |  | $ | 0.5 |
(A) | In addition to the $2.4 million and $3.5 million of net periodic benefit cost recognized during the three months ended June 30, 2018 and 2017, respectively, the Company recognized the following:
• | an increase in pension expense during the three months ended June 30, 2018 and 2017 of $3.8 million and $2.9 million, respectively, to maintain the allowable amount to be recovered for pension expense in the Oklahoma jurisdiction, which are included in the Pension tracker regulatory liability (see Note 1); and
• | a deferral of pension expense during the three months ended June 30, 2017 of $2.3 million related to the Arkansas jurisdictional portion of the pension settlement charge of $22.4 million in 2013.
(B) | In addition to the $4.8 million and $7.0 million of net periodic benefit cost recognized during the six months ended June 30, 2018 and 2017, respectively, the Company recognized the following:
• | an increase in pension expense during the six months ended June 30, 2018 and 2017 of $7.8 million and $5.8 million, respectively, to maintain the allowable amount to be recovered for pension expense in the Oklahoma jurisdiction, which are included in the Pension tracker regulatory liability (see Note 1); and
• | a deferral of pension expense during the six months ended June 30, 2017 of $2.3 million related to the Arkansas jurisdictional portion of the pension settlement charge of $22.4 million in 2013.
| Postretirement Benefit Plans
| Three Months Ended | Six Months Ended
| June 30, | June 30,
(In millions) | 2018 (B) | 2017 (B) | 2018 (C) | 2017 (C)
Service cost | $ | 0.1 |  | $ | 0.2 |  | $ | 0.2 |  | $ | 0.4 |
Interest cost | 1.3 |  | 2.1 |  | 2.6 |  | 4.3 |
Expected return on plan assets | (0.5 | ) | (0.5 | ) | (1.0 | ) | (1.1 | )
Amortization of net loss | 0.9 |  | 0.2 |  | 1.9 |  | 0.8 |
Amortization of unrecognized prior service cost (A) | (2.1 | ) | — |  | (4.2 | ) | — |
Total net periodic benefit cost | (0.3 | ) | 2.0 |  | (0.5 | ) | 4.4 |
Less: Amount paid by unconsolidated affiliates | (0.1 | ) | 0.2 |  | (0.2 | ) | 0.6 |
Net periodic benefit cost | $ | (0.2 | ) | $ | 1.8 |  | $ | (0.3 | ) | $ | 3.8 |
(A) | Unamortized prior service cost is amortized on a straight-line basis over the average remaining service period to the first eligibility age of participants who are expected to receive a benefit and are active at the date of the plan amendment.
(B) | In addition to the $0.2 million of net periodic benefit income and $1.8 million of net periodic benefit cost recognized during the three months ended June 30, 2018 and 2017, respectively, the Company recognized an increase in postretirement medical expense in the three months ended June 30, 2018 and 2017 of $2.2 million and $1.0 million, respectively, to maintain the allowable amount to be recovered for postretirement medical expense in the Oklahoma jurisdiction which are included in the Pension tracker regulatory liability (see Note 1).
(C) | In addition to the $0.3 million of net periodic benefit income and $3.8 million of net periodic benefit cost recognized during the six months ended June 30, 2018 and 2017, respectively, the Company recognized an increase in postretirement medical expense in the six months ended June 30, 2018 and 2017 of $4.3 million and $2.1 million, respectively, to maintain the allowable amount to be recovered for postretirement medical expense in the Oklahoma jurisdiction which are included in the Pension tracker regulatory liability (see Note 1).
Schedule of Capitalized Pension and Postretirement Cost [Table Text Block] | As required by ASU 2017-07, the Company only capitalizes the service cost component of net benefit cost, beginning in the first quarter of 2018. Prior year capitalized amounts were not adjusted, as this change was implemented on a prospective basis.
| Three Months Ended | Six Months Ended
| June 30, | June 30,
(In millions) | 2018 | 2017 | 2018 | 2017
Capitalized portion of net periodic pension benefit cost | $ | 0.9 |  | $ | 1.2 |  | $ | 1.9 |  | $ | 2.3 |
Capitalized portion of net periodic postretirement benefit cost | $ | 0.1 |  | $ | 0.5 |  | $ | 0.1 |  | $ | 1.2 |

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- Definition Tabular disclosure of one or more of the entity's defined benefit pension plans or one or more other defined benefit postretirement plans, separately for pension plans and other postretirement benefit plans including the entity's schedule of fair value of plan assets for defined benefit or other postretirement plans. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=112275985&loc=d3e4179-114921 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e2439-114920 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e2410-114920 Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e2417-114920 + Details
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