Commitments and Contingencies - Operating Leases (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Coal, Gas and Transportation Commitments
Out-of-market contracts, net		$ 121	$ 129
Future commitments under coal, gas and transportation contractual agreements
Future minimum sublease rentals		29
Powerton and Joliet
Future commitments under coal, gas and transportation contractual agreements
2019		1
2020		1
2021		3
2022		6
2023		6
Thereafter		222
Total		239
Other Leased Property
Coal, Gas and Transportation Commitments
Lease expense		66	$ 69	$ 85
Future commitments under coal, gas and transportation contractual agreements
2019		60
2020		55
2021		43
2022		40
2023		39
Thereafter		95
Total	[1]	$ 332
EME Project Financings
Coal, Gas and Transportation Commitments
Leased Interest		100.00%
Out-of-market contracts, net				$ 159
Lease expense		$ 14

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