Financing Receivables (Schedule Of Assets Under Lease Receivables) (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) MW
Powerton Station Units 5 And 6 [Member]
Financing Receivable, Recorded Investment [Line Items]
Lease Receivable, Asset Location	IL
Lease Receivable, Gross Investment | $	$ 134
Lease Receivable, % Owned	64.00%
Lease Receivable, Total, MW | MW	1,538
Lease Receivable, Asset, Fuel Type	Coal
Lease Receivable, Counterparties' S&P Credit Ratings	BB-
Lease Receivable, Counterparty	NRG Energy, Inc.
Joliet Station Units 7 And 8 [Member]
Financing Receivable, Recorded Investment [Line Items]
Lease Receivable, Asset Location	IL
Lease Receivable, Gross Investment | $	$ 83
Lease Receivable, % Owned	64.00%
Lease Receivable, Total, MW | MW	1,036
Lease Receivable, Asset, Fuel Type	Gas
Lease Receivable, Counterparties' S&P Credit Ratings	BB-
Lease Receivable, Counterparty	NRG Energy, Inc.
Keystone Station Units 1 And 2 [Member]
Financing Receivable, Recorded Investment [Line Items]
Lease Receivable, Asset Location	PA
Lease Receivable, Gross Investment | $	$ 55
Lease Receivable, % Owned	17.00%
Lease Receivable, Total, MW | MW	1,711
Lease Receivable, Asset, Fuel Type	Coal
Lease Receivable, Counterparties' S&P Credit Ratings	CCC (A)
Lease Receivable, Counterparty	REMA
Conemaugh Station Units 1 And 2 [Member]
Financing Receivable, Recorded Investment [Line Items]
Lease Receivable, Asset Location	PA
Lease Receivable, Gross Investment | $	$ 55
Lease Receivable, % Owned	17.00%
Lease Receivable, Total, MW | MW	1,711
Lease Receivable, Asset, Fuel Type	Coal
Lease Receivable, Counterparties' S&P Credit Ratings	CCC (A)
Lease Receivable, Counterparty	REMA
Shawville Station Units 1, 2, 3 And 4 [Member]
Financing Receivable, Recorded Investment [Line Items]
Lease Receivable, Asset Location	PA
Lease Receivable, Gross Investment | $	$ 99
Lease Receivable, % Owned	100.00%
Lease Receivable, Total, MW | MW	596
Lease Receivable, Asset, Fuel Type	Gas
Lease Receivable, Counterparties' S&P Credit Ratings	CCC (A)
Lease Receivable, Counterparty	REMA