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Benefit Plans - Rollforward of level 3 plan assets (Details) - Pension benefits - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Change in pension plan assets |  |  |
Fair value of plan assets, beginning of year | $ 6,284 | $ 5,650 |
Fair value of plan assets, end of year | 5,299 | 6,284 | $ 5,650
Significant unobservable inputs (Level 3) |  |  |
Change in pension plan assets |  |  |
Fair value of plan assets, beginning of year | 39 | 10 | 18
Actual return on plan assets: Relating to assets sold during the period | 0 | 0 | (1)
Actual return on plan assets: Relating to assets still held at the reporting date | 3 | 0 | 0
Purchases, sales and settlements, net | (5) | 29 | (12)
Net transfers in and/or (out) of Level 3 | (32) | 0 | 5
Fair value of plan assets, end of year | 5 | 39 | 10
Significant unobservable inputs (Level 3) | Equity securities |  |  |
Change in pension plan assets |  |  |
Fair value of plan assets, beginning of year | 29 | 0 | 1
Actual return on plan assets: Relating to assets sold during the period | 0 | 0 | (1)
Actual return on plan assets: Relating to assets still held at the reporting date | 3 | 0 | 0
Purchases, sales and settlements, net | 0 | 29 | 0
Net transfers in and/or (out) of Level 3 | (32) | 0 | 0
Fair value of plan assets, end of year | 0 | 29 | 0
Significant unobservable inputs (Level 3) | Municipal |  |  |
Change in pension plan assets |  |  |
Fair value of plan assets, beginning of year |  | 0 | 7
Actual return on plan assets: Relating to assets sold during the period |  |  | 0
Actual return on plan assets: Relating to assets still held at the reporting date |  |  | 0
Purchases, sales and settlements, net |  |  | (7)
Net transfers in and/or (out) of Level 3 |  |  | 0
Fair value of plan assets, end of year |  |  | 0
Significant unobservable inputs (Level 3) | Corporate |  |  |
Change in pension plan assets |  |  |
Fair value of plan assets, beginning of year | 10 | 10 | 10
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 | (5) | 0 | (5)
Net transfers in and/or (out) of Level 3 | 0 | 0 | 5
Fair value of plan assets, end of year | $ 5 | $ 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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(02)(A) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -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 + 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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(02)(A) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -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 + 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 Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/exampleRef -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 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 6: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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- Definition Amount, measured using unobservable input, of increase (decrease) in plan asset of defined benefit plan from purchase, sale and settlement of trade associated with underlying investment. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(02)(B) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -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 + 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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(02)(C) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -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 + Details
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