Debt and Other Obligations (Debt Purchases and Repayments) (Details) - USD ($) $ in Millions		3 Months Ended																12 Months Ended
		Dec. 31, 2018	[3]	Sep. 30, 2018	[3]	Jun. 30, 2018	[3]	Mar. 31, 2018	[3]	Dec. 31, 2017	[3]	Sep. 30, 2017	[3]	Jun. 30, 2017	[3]	Mar. 31, 2017	[3]	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Debt Instrument [Line Items]
Principal Amount																		$ 2,250		$ 0		$ 4,026
Cash Paid																		2,346	[1]	0		4,045	[2]
Gains (losses) on retirement of long-term obligations		$ 0		$ (32)		$ (3)		$ (71)		$ 0		$ 0		$ 0		$ (4)		(106)	[4]	(4)	[5]	(52)	[6]
2012 Credit Facility [Member]
Debt Instrument [Line Items]
Principal Amount																						0
Cash Paid	[2]																					0
Gains (losses) on retirement of long-term obligations	[6]																					(2)
Fixed Rate Securitized Debt January 2010 Tower Revenue Notes 3 [Member]
Debt Instrument [Line Items]
Principal Amount																		1,250
Cash Paid	[1]																	1,318
Gains (losses) on retirement of long-term obligations	[4]																	(71)
2016 Term Loan A [Member]
Debt Instrument [Line Items]
Principal Amount																		0		0
Cash Paid																		0	[1]	0
Gains (losses) on retirement of long-term obligations																		(3)	[4]	$ (4)	[5]
Variable Rate 2012 Term Loans Tranche A [Member]
Debt Instrument [Line Items]
Principal Amount																						629
Cash Paid	[2]																					629
Gains (losses) on retirement of long-term obligations	[6]																					(2)
Variable Rate 2012 Term Loans Tranche B [Member]
Debt Instrument [Line Items]
Principal Amount																						2,247
Cash Paid	[2]																					2,247
Gains (losses) on retirement of long-term obligations	[6]																					(27)
Fixed Rate Securitized Debt January 2010 Tower Revenue Notes 2 [Member]
Debt Instrument [Line Items]
Principal Amount																						350
Cash Paid	[2]																					353
Gains (losses) on retirement of long-term obligations	[6]																					(3)
Fixed Rate Securitized Debt August 2010 Tower Revenue Notes 5 [Member]
Debt Instrument [Line Items]
Principal Amount																						300
Cash Paid	[2]																					307
Gains (losses) on retirement of long-term obligations	[6]																					(8)
Fixed Rate Securitized Debt August 2010 Tower Revenue Notes 6 [Member]
Debt Instrument [Line Items]
Principal Amount																		1,000
Cash Paid	[1]																	1,028
Gains (losses) on retirement of long-term obligations	[4]																	$ (32)
2.381% Secured Notes [Member]
Debt Instrument [Line Items]
Principal Amount																						500
Cash Paid	[2]																					509
Gains (losses) on retirement of long-term obligations	[6]																					$ (10)

[1] Exclusive of accrued interest.

[2] Exclusive of accrued interest.

[3] The sum of quarterly information may not agree to year to date information due to rounding.

[4] Inclusive of the write off of the respective deferred financing costs.

[5] The losses related to write off of deferred financing costs

[6] Inclusive of the write off of the respective deferred financing costs.