Fair Value Measurements - Additional Information (Details) shares in Millions, $ in Millions		3 Months Ended	9 Months Ended	12 Months Ended
		Sep. 30, 2018 USD ($) interest_rate_swap_agreements	Sep. 30, 2018 USD ($) shares interest_rate_swap_agreements	Sep. 30, 2019 USD ($)	Dec. 31, 2017 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Gross		$ 9,090	$ 9,090
Debt Instrument, Convertible, Terms of Conversion Feature			The CEC Convertible Notes are convertible at the option of holders into a number of shares of CEC common stock that is equal to approximately 0.139 shares of CEC common stock per $1.00 principal amount of CEC Convertible Notes, which is equal to an initial conversion price of $7.19 per share. If all the shares were issued on the Effective Date, they would have represented approximately 17.9% of the shares of CEC common stock outstanding on a fully diluted basis. The holders of the CEC Convertible Notes can convert them at any time after issuance. CEC can convert the CEC Convertible Notes beginning in October 2020 if the last reported sale price of CEC common stock equals or exceeds 140% of the conversion price for the CEC Convertible Notes in effect on each of at least 20 trading days during any 30 consecutive trading day period. As of September 30, 2018, an immaterial amount of the CEC Convertible Notes were converted into shares of CEC common stock. An aggregate of 156 million shares of CEC common stock are issuable upon conversion of the CEC Convertible Notes. As of September 30, 2018, the remaining life of the CEC Convertible Notes is 6 years.
Description of Interest Rate Derivative Activities			During the nine months ended September 30, 2018, we entered into six interest rate swap agreements to fix the interest rate on $2.0 billion of variable rate debt.
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Gain (Loss) Included in Other Comprehensive Income (Loss)		$ 14	$ 31
Fair Value, Inputs, Level 3 [Member] | Convertible Debt [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Convertible, Number of Equity Instruments | shares			156
Debt Instrument, Interest Rate, Stated Percentage		6.00%	6.00%
Convertible Debt [Member] | Unsecured Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Gross		$ 1,082	$ 1,082		$ 1,100
Debt Instrument, Face Amount		$ 1,100	$ 1,100
Debt Instrument, Interest Rate, Stated Percentage	[1]	5.00%	5.00%
Designated as Hedging Instrument [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of Interest Rate Derivatives Held | interest_rate_swap_agreements		10	10
Derivative, Notional Amount		$ 3,000	$ 3,000
Measurement Input, Expected Term [Member] | Fair Value, Inputs, Level 3 [Member] | Convertible Debt [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Term			7 years
Measurement Input, Discount Rate [Member] | Fair Value, Inputs, Level 3 [Member] | Convertible Debt [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Interest Rate During Period			5.00%
Measurement Input, Price Volatility [Member] | Fair Value, Inputs, Level 3 [Member] | Convertible Debt [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Assumptions, Expected Term (Deprecated 2018-01-31)		0.30	0.30
Measurement Input, Risk Free Interest Rate [Member] | Fair Value, Inputs, Level 3 [Member] | Convertible Debt [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Assumptions, Expected Term (Deprecated 2018-01-31)		0.030	0.030
Scenario, Forecast [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, before Tax				$ 8

[1] (1) Interest rate is fixed, except where noted.