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

[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] See Note 17 for additional information about CGP’s debt.