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Benefit Plans - Rollforward of level 3 plan assets (Details) - Pension benefits - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Change in pension plan assets |  |  |
Fair value of plan assets, beginning of year | $ 5,650 | $ 5,353 |
Fair value of plan assets, end of year | 6,284 | 5,650 | $ 5,353
Significant unobservable inputs (Level 3) |  |  |
Change in pension plan assets |  |  |
Fair value of plan assets, beginning of year | 10 | 18 | 27
Actual return on plan assets: Relating to assets sold during the period | 0 | (1) | 1
Actual return on plan assets: Relating to assets still held at the reporting date | 0 | 0 | (1)
Purchases, sales and settlements, net | 29 | (12) | (7)
Net transfers in and/or (out) of Level 3 | 0 | 5 | (2)
Fair value of plan assets, end of year | 39 | 10 | 18
Significant unobservable inputs (Level 3) | Equity securities |  |  |
Change in pension plan assets |  |  |
Fair value of plan assets, beginning of year | 0 | 1 | 1
Actual return on plan assets: Relating to assets sold during the period | 0 | (1) | 1
Actual return on plan assets: Relating to assets still held at the reporting date | 0 | 0 | (1)
Purchases, sales and settlements, net | 29 | 0 | 0
Net transfers in and/or (out) of Level 3 | 0 | 0 | 0
Fair value of plan assets, end of year | 29 | 0 | 1
Significant unobservable inputs (Level 3) | Municipal |  |  |
Change in pension plan assets |  |  |
Fair value of plan assets, beginning of year | 0 | 7 | 14
Actual return on plan assets: Relating to assets sold during the period |  | 0 | 0
Actual return on plan assets: Relating to assets still held at the reporting date |  | 0 | 0
Purchases, sales and settlements, net |  | (7) | (7)
Net transfers in and/or (out) of Level 3 |  | 0 | 0
Fair value of plan assets, end of year |  | 0 | 7
Significant unobservable inputs (Level 3) | Corporate |  |  |
Change in pension plan assets |  |  |
Fair value of plan assets, beginning of year | 10 | 10 | 12
Actual return on plan assets: Relating to assets sold during the period | 0 | 0 | 0
Actual return on plan assets: Relating to assets still held at the reporting date | 0 | 0 | 0
Purchases, sales and settlements, net | 0 | (5) | 0
Net transfers in and/or (out) of Level 3 | 0 | 5 | (2)
Fair value of plan assets, end of year | $ 10 | $ 10 | $ 10

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- Definition Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets sold. + 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 (d)(5)(iv)(02)(A) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets still held. + 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 (d)(5)(iv)(02)(A) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
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- Definition Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees. + 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 (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from purchases, sales and settlements. + 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 (d)(5)(iv)(02)(B) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan, for assets transferred into (out of) Level 3 of fair value hierarchy. Includes, but is not limited to, transfer due to change in observability of significant inputs. + 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 (d)(5)(iv)(02)(C) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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