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Commitments and Contingencies - Operating Leases (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Coal, Gas and Transportation Commitments |  |  |  |
Out-of-market contracts, net |  | $ 207 | $ 230 |
Future commitments under coal, gas and transportation contractual agreements |  |  |  |
Future minimum sublease rentals |  | 49 |  |
Powerton and Joliet |  |  |  |
Future commitments under coal, gas and transportation contractual agreements |  |  |  |
2018 |  | 1 |  |
2019 |  | 1 |  |
2020 |  | 1 |  |
2021 |  | 3 |  |
2022 |  | 6 |  |
Thereafter |  | 228 |  |
Total |  | 240 |  |
Other Leased Property |  |  |  |
Coal, Gas and Transportation Commitments |  |  |  |
Lease expense |  | 81 | $ 96 | $ 97
Future commitments under coal, gas and transportation contractual agreements |  |  |  |
2018 |  | 78 |  |
2019 |  | 80 |  |
2020 |  | 75 |  |
2021 |  | 65 |  |
2022 |  | 64 |  |
Thereafter |  | 479 |  |
Total | [1] | $ 841 |  |
EME Project Financings |  |  |  |
Coal, Gas and Transportation Commitments |  |  |  |
Leased Interest |  | 100.00% |  |
Out-of-market contracts, net |  |  |  | $ 159
Lease expense |  | $ 14 |  |
[1] | Amounts in the table exclude future sublease income of $49 million associated with long-term leases for office locations.

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- Definition Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 10 -Section 55 -Paragraph 40 -Subparagraph (Note 3) -URI http://asc.fasb.org/extlink&oid=82846649&loc=d3e38371-112697 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41502-112717 + Details
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