Debt - Annual Estimated Debt Service Requirements (Details) - USD ($) $ in Millions	Mar. 31, 2017	[1]	Dec. 31, 2016	[2]
Other Commitments [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	$ 36		$ 89
Long-term Debt, Maturities, Repayments of Principal in Year Two	50		50
Long-term Debt, Maturities, Repayments of Principal in Year Three	222		223
Long-term Debt, Maturities, Repayments of Principal in Year Four	3,650		3,650
Long-term Debt, Maturities, Repayments of Principal in Year Five	2,249		2,249
Long-term Debt, Maturities, Repayments of Principal after Year Five	687		687
Long-term debt, Maturities, Repayments of Principal, Total	6,894		6,948
Contractual Obligation, Due in Next Fiscal Year	526		659
Contractual Obligation, Due in Second Year	630		630
Contractual Obligation, Due in Third Year	812		813
Contractual Obligation, Due in Fourth Year	4,170		4,170
Contractual Obligation, Due in Fifth Year	2,469		2,469
Contractual Obligation, Due after Fifth Year	727		727
Contractual Obligation, Total	9,334		9,468
Interest Expense [Member]
Other Commitments [Line Items]
Other Commitment, Due in Next Twelve Months	490		570
Other Commitment, Due in Second Year	580		580
Other Commitment, Due in Third Year	590		590
Other Commitment, Due in Fourth Year	520		520
Other Commitment, Due in Fifth Year	220		220
Other Commitment, Due after Fifth Year	40		40
Other Commitment, Total	2,440		2,520
CERP
Other Commitments [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	20		68
Long-term Debt, Maturities, Repayments of Principal in Year Two	25		25
Long-term Debt, Maturities, Repayments of Principal in Year Three	25		25
Long-term Debt, Maturities, Repayments of Principal in Year Four	3,350		3,350
Long-term Debt, Maturities, Repayments of Principal in Year Five	1,150		1,150
Long-term Debt, Maturities, Repayments of Principal after Year Five	0		0
Long-term debt, Maturities, Repayments of Principal, Total	4,570		4,618
Contractual Obligation, Due in Next Fiscal Year	360		458
Contractual Obligation, Due in Second Year	415		415
Contractual Obligation, Due in Third Year	425		425
Contractual Obligation, Due in Fourth Year	3,710		3,710
Contractual Obligation, Due in Fifth Year	1,280		1,280
Contractual Obligation, Due after Fifth Year	0		0
Contractual Obligation, Total	6,190		6,288
CERP | Interest Expense [Member]
Other Commitments [Line Items]
Other Commitment, Due in Next Twelve Months	340		390
Other Commitment, Due in Second Year	390		390
Other Commitment, Due in Third Year	400		400
Other Commitment, Due in Fourth Year	360		360
Other Commitment, Due in Fifth Year	130		130
Other Commitment, Due after Fifth Year	0		0
Other Commitment, Total	1,620		1,670
CGP
Other Commitments [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	16		21
Long-term Debt, Maturities, Repayments of Principal in Year Two	25		25
Long-term Debt, Maturities, Repayments of Principal in Year Three	197		198
Long-term Debt, Maturities, Repayments of Principal in Year Four	300		300
Long-term Debt, Maturities, Repayments of Principal in Year Five	1,099		1,099
Long-term Debt, Maturities, Repayments of Principal after Year Five	687		687
Long-term debt, Maturities, Repayments of Principal, Total	2,324		2,330
Contractual Obligation, Due in Next Fiscal Year	166		201
Contractual Obligation, Due in Second Year	215		215
Contractual Obligation, Due in Third Year	387		388
Contractual Obligation, Due in Fourth Year	460		460
Contractual Obligation, Due in Fifth Year	1,189		1,189
Contractual Obligation, Due after Fifth Year	727		727
Contractual Obligation, Total	3,144		3,180
CGP | Interest Expense [Member]
Other Commitments [Line Items]
Other Commitment, Due in Next Twelve Months	150		180
Other Commitment, Due in Second Year	190		190
Other Commitment, Due in Third Year	190		190
Other Commitment, Due in Fourth Year	160		160
Other Commitment, Due in Fifth Year	90		90
Other Commitment, Due after Fifth Year	40		40
Other Commitment, Total	$ 820		$ 850

[1] Debt principal payments are estimated amounts based on maturity dates and potential borrowings under our revolving credit facility. Interest payments are estimated based on the forward-looking LIBOR curve. Actual payments may differ from these estimates.

[2] Debt principal payments are estimated amounts based on maturity dates and potential borrowings under our revolving credit facility. Interest payments are estimated based on the forward-looking LIBOR curve. Actual payments may differ from these estimates.