Debt - Annual Estimated Debt Service Requirements (Details) $ in Millions	Dec. 31, 2018 USD ($) interest_rate_swap_agreements
Other Commitments [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	$ 164
Long-term Debt, Maturities, Repayments of Principal in Year Two	64
Long-term Debt, Maturities, Repayments of Principal in Year Three	64
Long-term Debt, Maturities, Repayments of Principal in Year Four	64
Long-term Debt, Maturities, Repayments of Principal in Year Five	64
Long-term Debt, Maturities, Repayments of Principal after Year Five	8,655
Long-term debt, Maturities, Repayments of Principal, Total	9,075
Contractual Obligation, Due in Next Fiscal Year	644	[1]
Contractual Obligation, Due in Second Year	534	[1]
Contractual Obligation, Due in Third Year	534	[1]
Contractual Obligation, Due in Fourth Year	524	[1]
Contractual Obligation, Due in Fifth Year	514	[1]
Contractual Obligation, Due after Fifth Year	9,195	[1]
Contractual Obligation, Total	11,945	[1]
Interest Expense [Member]
Other Commitments [Line Items]
Other Commitment, Due in Next Twelve Months	480
Other Commitment, Due in Second Year	470
Other Commitment, Due in Third Year	470
Other Commitment, Due in Fourth Year	460
Other Commitment, Due in Fifth Year	450
Other Commitment, Due after Fifth Year	540
Other Commitment, Total	$ 2,870
Designated as Hedging Instrument [Member]
Other Commitments [Line Items]
Number of Interest Rate Derivatives Held | interest_rate_swap_agreements	10

[1] Debt principal payments are estimated amounts based on maturity dates and potential borrowings under our revolving credit facilities. Interest payments are estimated based on the forward-looking LIBOR curve and include the estimated impact of the ten interest rate swap agreements (see Note 8). Actual payments may differ from these estimates.