Revenue Recognition - Disaggregation Of Revenue (Details) $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($)
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 3,901
Transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,912
Transferred over time \| Retail service
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,623
Transferred over time \| Inbound roaming
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	154
Transferred over time \| Other service
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	135
Transferred at point in time \| Equipment sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 989

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130543-203045

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130539-203045

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130545-203045

Name:	us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration