Fair Value Measurements - Additional Details (Details)	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($) count	Dec. 31, 2013 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Settlements	$ 17,000,000
Net Periodic Cash Settlements and Accrued Interest		$ 177,000,000	$ 172,000,000
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	7,000,000	17,000,000	34,000,000
Interest Expense	684,000,000	2,670,000,000	2,252,000,000
Tangible and intangible asset impairments	1,000,000	994,000,000	2,831,000,000
Reclassification out of Accumulated Other Comprehensive Income [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest Expense			$ 4,000,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability		6,000,000
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure		848,000,000
Fair Value, Measurements, Nonrecurring [Member] | Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure	$ 0
Tangible and intangible asset impairments		$ 642,000,000
Interest Rate Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative, Number of Instruments Held | count		8
Derivative, Notional Amount		$ 5,800,000,000