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Retirement Plans and Postretirement Benefit Plans (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2016 | Mar. 31, 2015
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract] |  |  |
Net periodic benefit cost |  | $ 4.0 | $ 4.6
Pension Plans, Defined Benefit [Member] |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract] |  |  |
Service cost |  | 4.4 | 4.5
Interest cost |  | 6.6 | 6.4
Expected return on plan assets |  | (10.5) | (11.8)
Amortization of net loss |  | 4.2 | 4.3
Amortization of unrecognized prior service cost | [1] | 0.0 | 0.1
Net periodic benefit cost |  | 4.7 | 3.5
Amount Attributable to Unconsolidated Affiliates |  | 1.3 | 1.1
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates | [2] | 3.4 | 2.4
Capitalized Portion of Net Periodic Benefit Cost |  | 1.2 | 0.8
Pension Plans, Defined Benefit [Member] | OKLAHOMA |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract] |  |  |
Additional Pension Expense to Meet State Requirements |  | 2.3 | 3.0
Restoration of Retirement Income Plan [Member] |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract] |  |  |
Service cost |  | 0.1 | 0.4
Interest cost |  | 0.1 | 0.2
Expected return on plan assets |  | 0.0 | 0.0
Amortization of net loss |  | 0.2 | 0.1
Amortization of unrecognized prior service cost | [1] | 0.0 | 0.0
Net periodic benefit cost |  | 0.4 | 0.7
Amount Attributable to Unconsolidated Affiliates |  | 0.0 | 0.0
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates | [2] | 0.4 | 0.7
Other Postretirement Benefit Plans, Defined Benefit [Member] |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract] |  |  |
Service cost |  | 0.3 | 0.5
Interest cost |  | 2.3 | 2.6
Expected return on plan assets |  | (0.6) | (0.6)
Amortization of net loss |  | 0.5 | 3.4
Amortization of unrecognized prior service cost | [1] | (2.2) | (4.1)
Net periodic benefit cost |  | 0.3 | 1.8
Amount Attributable to Unconsolidated Affiliates |  | 0.1 | 0.3
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates | [2] | 0.2 | 1.5
Capitalized Portion of Net Periodic Benefit Cost |  | 0.1 | 0.5
Other Postretirement Benefit Plans, Defined Benefit [Member] | OKLAHOMA |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract] |  |  |
Additional Postretirement Medical Expense to Meet State Requirements |  | $ 2.0 | $ 1.4
[1] | 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.
[2] | In addition to the $4.0 million and $4.6 million of net periodic benefit cost recognized during the three months ended March 31, 2016 and 2015, respectively, the Company recognized the following: •an increase in pension expense in the three months ended March 31, 2016 and 2015 of $2.3 million and $3.0 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 •an increase in postretirement medical expense in the three months ended March 31, 2016 and 2015 of $2.0 million and $1.4 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).

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- Definition Defined Benefit Plan, Net Periodic Benefit Cost, Net of Unconsolidated Affiliates + References No definition available. + Details
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- Definition The amount of the prior service cost or credit recognized in net periodic benefit cost relating to benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Prior Service Cost -URI http://asc.fasb.org/extlink&oid=49177896 + Details
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- Definition An amount calculated as a basis for determining the extent of delayed recognition of the effects of changes in the fair value of assets. The expected return on plan assets is determined based on the expected long-term rate of return on plan assets and the market-related value of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Expected Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512171 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(3) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920 + Details
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- Definition The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(2) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920 + Details
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- Definition The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920 + Details
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- Definition The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Service Cost (Component of Net Periodic Pension Cost) -URI http://asc.fasb.org/extlink&oid=6525008 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920 + Details
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