Guarantor Financial Information - Condensed Consolidating Statement of Comprehensive Income Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revenues
Revenues	$ 11,080	$ 10,455
Operating expenses
Selling, general and administrative	3,442	3,164
Depreciation and amortization	1,600	1,575
Total operating expense	9,604	9,173
Operating income	1,476	1,282
Other income (expense)
Interest expense	(179)	(251)
Interest expense to affiliates	(109)	(166)
Interest income	8	6
Other income (expense), net	7	10
Total other expense, net	(273)	(401)
Income before income taxes	1,203	881
Income tax expense	(295)	(210)
Earnings of subsidiaries	0	0
Net income	908	671
Other comprehensive income (loss), net of tax	(189)	(3)
Total comprehensive income	719	668
Service
Revenues
Revenues	8,277	7,806
Operating expenses
Cost of services and equipment sales	1,546	1,589
Product
Revenues
Revenues	2,516	2,353
Operating expenses
Cost of services and equipment sales	3,016	2,845
Other revenues
Revenues
Revenues	287	296
Consolidating and Eliminating Adjustments
Revenues
Revenues	(410)	(284)
Operating expenses
Selling, general and administrative	(328)	(229)
Depreciation and amortization	0	0
Total operating expense	(410)	(284)
Operating income	0	0
Other income (expense)
Interest expense	0	0
Interest expense to affiliates	5	5
Interest income	(5)	(5)
Other income (expense), net	0	0
Total other expense, net	0	0
Income before income taxes	0	0
Income tax expense	0	0
Earnings of subsidiaries	(2,026)	(1,701)
Net income	(2,026)	(1,701)
Other comprehensive income (loss), net of tax	124	6
Total comprehensive income	(1,902)	(1,695)
Consolidating and Eliminating Adjustments | Service
Revenues
Revenues	(314)	(221)
Operating expenses
Cost of services and equipment sales	(28)	0
Consolidating and Eliminating Adjustments | Product
Revenues
Revenues	(54)	(54)
Operating expenses
Cost of services and equipment sales	(54)	(55)
Consolidating and Eliminating Adjustments | Other revenues
Revenues
Revenues	(42)	(9)
Parent | Reportable Legal Entities
Revenues
Revenues	0	0
Operating expenses
Selling, general and administrative	0	0
Depreciation and amortization	0	0
Total operating expense	0	0
Operating income	0	0
Other income (expense)
Interest expense	0	0
Interest expense to affiliates	0	0
Interest income	0	0
Other income (expense), net	0	0
Total other expense, net	0	0
Income before income taxes	0	0
Income tax expense	0	0
Earnings of subsidiaries	908	671
Net income	908	671
Other comprehensive income (loss), net of tax	(189)	(3)
Total comprehensive income	719	668
Parent | Reportable Legal Entities | Service
Revenues
Revenues	0	0
Operating expenses
Cost of services and equipment sales	0	0
Parent | Reportable Legal Entities | Product
Revenues
Revenues	0	0
Operating expenses
Cost of services and equipment sales	0	0
Parent | Reportable Legal Entities | Other revenues
Revenues
Revenues	0	0
Issuer | Reportable Legal Entities
Revenues
Revenues	6	1
Operating expenses
Selling, general and administrative	1	0
Depreciation and amortization	0	0
Total operating expense	1	0
Operating income	5	1
Other income (expense)
Interest expense	(112)	(174)
Interest expense to affiliates	(109)	(166)
Interest income	5	6
Other income (expense), net	8	(32)
Total other expense, net	(208)	(366)
Income before income taxes	(203)	(365)
Income tax expense	0	0
Earnings of subsidiaries	1,111	1,036
Net income	908	671
Other comprehensive income (loss), net of tax	(189)	(3)
Total comprehensive income	719	668
Issuer | Reportable Legal Entities | Service
Revenues
Revenues	0	0
Operating expenses
Cost of services and equipment sales	0	0
Issuer | Reportable Legal Entities | Product
Revenues
Revenues	0	0
Operating expenses
Cost of services and equipment sales	0	0
Issuer | Reportable Legal Entities | Other revenues
Revenues
Revenues	6	1
Guarantor Subsidiaries | Reportable Legal Entities
Revenues
Revenues	10,702	10,143
Operating expenses
Selling, general and administrative	3,494	3,157
Depreciation and amortization	1,578	1,554
Total operating expense	9,438	8,955
Operating income	1,264	1,188
Other income (expense)
Interest expense	(20)	(29)
Interest expense to affiliates	(5)	(5)
Interest income	7	5
Other income (expense), net	(1)	42
Total other expense, net	(19)	13
Income before income taxes	1,245	1,201
Income tax expense	(261)	(199)
Earnings of subsidiaries	7	(6)
Net income	991	996
Other comprehensive income (loss), net of tax	65	(3)
Total comprehensive income	1,056	993
Guarantor Subsidiaries | Reportable Legal Entities | Service
Revenues
Revenues	7,859	7,487
Operating expenses
Cost of services and equipment sales	1,568	1,580
Guarantor Subsidiaries | Reportable Legal Entities | Product
Revenues
Revenues	2,570	2,407
Operating expenses
Cost of services and equipment sales	2,798	2,664
Guarantor Subsidiaries | Reportable Legal Entities | Other revenues
Revenues
Revenues	273	249
Non-Guarantor Subsidiaries | Reportable Legal Entities
Revenues
Revenues	782	595
Operating expenses
Selling, general and administrative	275	236
Depreciation and amortization	22	21
Total operating expense	575	502
Operating income	207	93
Other income (expense)
Interest expense	(47)	(48)
Interest expense to affiliates	0	0
Interest income	1	0
Other income (expense), net	0	0
Total other expense, net	(46)	(48)
Income before income taxes	161	45
Income tax expense	(34)	(11)
Earnings of subsidiaries	0	0
Net income	127	34
Other comprehensive income (loss), net of tax	0	0
Total comprehensive income	127	34
Non-Guarantor Subsidiaries | Reportable Legal Entities | Service
Revenues
Revenues	732	540
Operating expenses
Cost of services and equipment sales	6	9
Non-Guarantor Subsidiaries | Reportable Legal Entities | Product
Revenues
Revenues	0	0
Operating expenses
Cost of services and equipment sales	272	236
Non-Guarantor Subsidiaries | Reportable Legal Entities | Other revenues
Revenues
Revenues	$ 50	$ 55