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Derivative Financial Instruments - Summary of Economic Hedges (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Derivative [Line Items]        
Unrealized Gain (Loss) on Derivatives     $ (149) $ (100)
Unrealized Gain (Loss) on Derivatives and Commodity Contracts $ 73 $ 88 (161) (113)
Exelon Generation Co L L C [Member]        
Derivative [Line Items]        
Unrealized Gain (Loss) on Derivatives     (160) (112)
Unrealized Gain (Loss) on Derivatives and Commodity Contracts 73 88 (161) (113)
Operating Revenue [Member] | Exelon Generation Co L L C [Member]        
Derivative [Line Items]        
Unrealized Gain (Loss) on Derivatives and Commodity Contracts 52 187 (47) (366)
Purchased Power And Fuel [Member] | Exelon Generation Co L L C [Member]        
Derivative [Line Items]        
Unrealized Gain (Loss) on Derivatives and Commodity Contracts 21 (99) (114) 253
Foreign Exchange Contract [Member] | Operating Revenue [Member] | Exelon Generation Co L L C [Member]        
Derivative [Line Items]        
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net 0 (2) (2) (6)
Unrealized Gain (Loss) on Derivatives (3) (1) (6) (9)
Change in Unrealized Gain (Loss) on Hedged Item in Foreign Currency Fair Value Hedge (3) 1 (4) (3)
Foreign Exchange Contract [Member] | Purchased Power And Fuel [Member] | Exelon Generation Co L L C [Member]        
Derivative [Line Items]        
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net 0 0 0 0
Unrealized Gain (Loss) on Derivatives 0 0 0 0
Change in Unrealized Gain (Loss) on Hedged Item in Foreign Currency Fair Value Hedge 0 0 0 0
Foreign Exchange Contract [Member] | Interest Expense [Member]        
Derivative [Line Items]        
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net 0 (2) (2) (6)
Unrealized Gain (Loss) on Derivatives (3) (1) (6) (9)
Change in Unrealized Gain (Loss) on Hedged Item in Foreign Currency Fair Value Hedge (3) 1 (4) (3)
Foreign Exchange Contract [Member] | Interest Expense [Member] | Exelon Generation Co L L C [Member]        
Derivative [Line Items]        
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net 0 (2) (2) (6)
Unrealized Gain (Loss) on Derivatives (3) (1) (6) (9)
Change in Unrealized Gain (Loss) on Hedged Item in Foreign Currency Fair Value Hedge (3) 1 (4) (3)
Commodity Contract [Member] | Operating Revenue [Member] | Exelon Generation Co L L C [Member]        
Derivative [Line Items]        
Unrealized Gain (Loss) on Commodity Contracts 132 280 123 127
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net (77) (92) (164) (484)
Unrealized Gain (Loss) on Derivatives 55 188 (41) (357)
Commodity Contract [Member] | Purchased Power And Fuel [Member] | Exelon Generation Co L L C [Member]        
Derivative [Line Items]        
Unrealized Gain (Loss) on Commodity Contracts 45 (73) (153) 36
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net (24) (26) 39 217
Unrealized Gain (Loss) on Derivatives 21 (99) (114) 253
Commodity Contract [Member] | Interest Expense [Member]        
Derivative [Line Items]        
Unrealized Gain (Loss) on Commodity Contracts 177 207 (30) 163
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net (101) (118) (125) (267)
Unrealized Gain (Loss) on Derivatives 76 89 (155) (104)
Commodity Contract [Member] | Interest Expense [Member] | Exelon Generation Co L L C [Member]        
Derivative [Line Items]        
Unrealized Gain (Loss) on Commodity Contracts 177 207 (30) 163
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net (101) (118) (125) (267)
Unrealized Gain (Loss) on Derivatives $ 76 $ 89 $ (155) $ (104)