Fair Value Measurements (Assets and Liabilities Measured on a Recurring Basis) (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2015 pipeline contract	Dec. 31, 2016 USD ($)
Derivatives:
Derivative assets	$ 1,988		$ 1,988			$ 2,235
Derivatives:
Derivative liability	771		771			881
Number of long-term contracted natural gas pipeline assets acquired | pipeline					7
Threshold for payment of contingent holdback, number of executed contracts | contract					1
Revaluation of contingent consideration	0	$ (101)	0	$ (118)
Fair value measurements made on a recurring basis [Member]
Cash equivalents:
Equity securities	999		999			982
Special use funds:
Equity securities	3,285		3,285			2,913
U.S. Government and municipal bonds	542		542			466
Corporate debt securities	773		773			764
Mortgage-backed securities	452		452			498
Other debt securities	126		126			81
Other investments:
Equity Securities	35		35			35
Debt securities	141		141			161
Fair value measurements made on a recurring basis [Member] | Commodity Contract [Member]
Derivatives:
Derivative asset, netting	(2,196)		(2,196)			(2,652)
Derivative assets	1,885		1,885			1,938
Derivatives:
Derivative liability, netting	(2,143)		(2,143)			(2,485)
Derivative liability	403		403			483
Fair value measurements made on a recurring basis [Member] | Interest Rate Contract [Member]
Derivatives:
Derivative asset, netting	16		16			8
Derivative assets	92		92			296
Derivatives:
Derivative liability, netting	16		16			8
Derivative liability	315		315			292
Fair value measurements made on a recurring basis [Member] | Foreign Currency Swap [Member]
Derivatives:
Derivative asset, netting	11		11			0
Derivative assets	11		11			1
Derivatives:
Derivative liability, netting	11		11			0
Derivative liability	53		53			106
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) [Member] | Fair value measurements made on a recurring basis [Member]
Cash equivalents:
Equity securities	999		999			982
Special use funds:
Equity securities	1,590		1,590			1,410
U.S. Government and municipal bonds	404		404			296
Corporate debt securities	1		1			1
Mortgage-backed securities	0		0			0
Other debt securities	0		0			0
Other investments:
Equity Securities	25		25			26
Debt securities	32		32			8
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) [Member] | Fair value measurements made on a recurring basis [Member] | Commodity Contract [Member]
Derivatives:
Derivative asset before netting	1,168		1,168			1,563
Derivatives:
Derivative liability before netting	1,144		1,144			1,476
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) [Member] | Fair value measurements made on a recurring basis [Member] | Interest Rate Contract [Member]
Derivatives:
Derivative asset before netting	0		0			0
Derivatives:
Derivative liability before netting	0		0			0
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) [Member] | Fair value measurements made on a recurring basis [Member] | Foreign Currency Swap [Member]
Derivatives:
Derivative asset before netting	0		0			0
Derivatives:
Derivative liability before netting	0		0			0
Significant Other Observable Inputs (Level 2) [Member] | Fair value measurements made on a recurring basis [Member]
Cash equivalents:
Equity securities	0		0			0
Special use funds:
Equity securities	1,695		1,695			1,503
U.S. Government and municipal bonds	138		138			170
Corporate debt securities	772		772			763
Mortgage-backed securities	452		452			498
Other debt securities	126		126			81
Other investments:
Equity Securities	10		10			9
Debt securities	109		109			153
Significant Other Observable Inputs (Level 2) [Member] | Fair value measurements made on a recurring basis [Member] | Commodity Contract [Member]
Derivatives:
Derivative asset before netting	1,503		1,503			1,827
Derivatives:
Derivative liability before netting	900		900			980
Significant Other Observable Inputs (Level 2) [Member] | Fair value measurements made on a recurring basis [Member] | Interest Rate Contract [Member]
Derivatives:
Derivative asset before netting	76		76			285
Derivatives:
Derivative liability before netting	171		171			171
Significant Other Observable Inputs (Level 2) [Member] | Fair value measurements made on a recurring basis [Member] | Foreign Currency Swap [Member]
Derivatives:
Derivative asset before netting	0		0			1
Derivatives:
Derivative liability before netting	42		42			106
Significant Unobservable Inputs (Level 3) [Member] | Fair value measurements made on a recurring basis [Member]
Cash equivalents:
Equity securities	0		0			0
Special use funds:
Equity securities	0		0			0
U.S. Government and municipal bonds	0		0			0
Corporate debt securities	0		0			0
Mortgage-backed securities	0		0			0
Other debt securities	0		0			0
Other investments:
Equity Securities	0		0			0
Debt securities	0		0			0
Significant Unobservable Inputs (Level 3) [Member] | Fair value measurements made on a recurring basis [Member] | Commodity Contract [Member]
Derivatives:
Derivative asset before netting	1,410		1,410			1,200
Derivatives:
Derivative liability before netting	502		502			512
Significant Unobservable Inputs (Level 3) [Member] | Fair value measurements made on a recurring basis [Member] | Interest Rate Contract [Member]
Derivatives:
Derivative asset before netting	0		0			3
Derivatives:
Derivative liability before netting	128		128			113
Significant Unobservable Inputs (Level 3) [Member] | Fair value measurements made on a recurring basis [Member] | Foreign Currency Swap [Member]
Derivatives:
Derivative asset before netting	0		0			0
Derivatives:
Derivative liability before netting	0		0			0
Other Current Assets [Member]
Derivatives:
Restricted cash	137		137			164
FPL [Member]
Derivatives:
Derivative assets	7		7			209
Derivatives:
Derivative liability	3		3			1
FPL [Member] | Fair value measurements made on a recurring basis [Member]
Cash equivalents:
Equity securities	112		112			120
Special use funds:
Equity securities	1,987		1,987			1,745
U.S. Government and municipal bonds	416		416			362
Corporate debt securities	542		542			547
Mortgage-backed securities	343		343			384
Other debt securities	114		114			70
FPL [Member] | Fair value measurements made on a recurring basis [Member] | Commodity Contract [Member]
Derivatives:
Derivative asset, netting	(4)		(4)			(3)
Derivative assets	7		7			209
Derivatives:
Derivative liability, netting	(4)		(4)			(3)
Derivative liability	3		3			1
FPL [Member] | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) [Member] | Fair value measurements made on a recurring basis [Member]
Cash equivalents:
Equity securities	112		112			120
Special use funds:
Equity securities	442		442			373
U.S. Government and municipal bonds	301		301			221
Corporate debt securities	0		0			0
Mortgage-backed securities	0		0			0
Other debt securities	0		0			0
FPL [Member] | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) [Member] | Fair value measurements made on a recurring basis [Member] | Commodity Contract [Member]
Derivatives:
Derivative asset before netting	0		0			0
Derivatives:
Derivative liability before netting	0		0			0
FPL [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value measurements made on a recurring basis [Member]
Cash equivalents:
Equity securities	0		0			0
Special use funds:
Equity securities	1,545		1,545			1,372
U.S. Government and municipal bonds	115		115			141
Corporate debt securities	542		542			547
Mortgage-backed securities	343		343			384
Other debt securities	114		114			70
FPL [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value measurements made on a recurring basis [Member] | Commodity Contract [Member]
Derivatives:
Derivative asset before netting	10		10			208
Derivatives:
Derivative liability before netting	5		5			1
FPL [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value measurements made on a recurring basis [Member]
Cash equivalents:
Equity securities	0		0			0
Special use funds:
Equity securities	0		0			0
U.S. Government and municipal bonds	0		0			0
Corporate debt securities	0		0			0
Mortgage-backed securities	0		0			0
Other debt securities	0		0			0
FPL [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value measurements made on a recurring basis [Member] | Commodity Contract [Member]
Derivatives:
Derivative asset before netting	1		1			4
Derivatives:
Derivative liability before netting	2		2			3
FPL [Member] | Other Current Assets [Member]
Derivatives:
Restricted cash	$ 99		$ 99			$ 120