Debt and Other Obligations (Debt Purchases and Repayments) (Details) - USD ($) $ in Millions		3 Months Ended																6 Months Ended		9 Months Ended		12 Months Ended
		Dec. 31, 2019	[2]	Sep. 30, 2019	[2]	Jun. 30, 2019	[2]	Mar. 31, 2019	[2]	Dec. 31, 2018	[2]	Sep. 30, 2018	[2]	Jun. 30, 2018	[2]	Mar. 31, 2018	[2]	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Debt Instrument [Line Items]
Principal Amount																						$ 12		$ 2,250		$ 0
Cash Paid																						12	[1]	2,346	[1]	0
Gains (losses) on retirement of long-term obligations		$ 0		$ 0		$ (1)		$ (1)		$ 0		$ (32)		$ (3)		$ (71)		$ (2)	$ (74)	$ (2)	$ (106)	(2)	[3]	(106)	[3]	(4)	[4]
Fixed Rate Debt 2009 Securitized Notes A-1 [Member]
Debt Instrument [Line Items]
Principal Amount																						12
Cash Paid	[1]																					12
Gains (losses) on retirement of long-term obligations	[3]																					(1)
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	[3]																							(71)
2016 Term Loan A [Member]
Debt Instrument [Line Items]
Principal Amount																						0		0		0
Cash Paid																						0	[1]	0	[1]	0
Gains (losses) on retirement of long-term obligations																						$ (1)	[3]	(3)	[3]	$ (4)	[4]
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	[3]																							$ (32)

[1] Exclusive of accrued interest.

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

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

[4] The losses represent write-off of deferred financing costs.