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Retirement Plans and Other Postretirement Benefits - Narrative (Details) - USD ($) |  | 1 Months Ended | 3 Months Ended | 9 Months Ended
--- | --- | --- | --- | ---
Sep. 30, 2014 | Jul. 31, 2012 | Sep. 30, 2015 | Sep. 30, 2016 | Sep. 30, 2015
Regulatory Assets |  |  |  |  |
Regulatory asset amortization period |  | 3 years |  |  |
Amortization of regulatory asset |  |  | $ 2,000,000 |  | $ 4,000,000
Pension Benefits |  |  |  |  |
Contributions |  |  |  |  |
Voluntary employer contributions to pension plan |  |  |  | $ 100,000,000 |
Minimum employer contributions for the next three years |  |  |  | 0 |
Maximum employer contributions for the next two years (up to) |  |  |  | 300,000,000 |
Other Benefits |  |  |  |  |
Other Postretirement Benefit Plan Remeasurement |  |  |  |  |
Other postretirement plan benefit remeasurement, amount seeking approval to move to separate account to Pay Union employee medical costs | $ 140,000,000 |  |  |  |
Contributions |  |  |  |  |
2015 |  |  |  | 1,000,000 |
2016 (up to) |  |  |  | 1,000,000 |
2017 (up to) |  |  |  | $ 1,000,000 |

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- Definition The expense charged against earnings during the period to allocate the capitalized costs of regulatory assets over the periods expected to benefit from such costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.3) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 + Details
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