Recent Accounting Pronouncements (Policies)	6 Months Ended
Recent Accounting Pronouncements (Policies)	Jun. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Recent Accounting Pronouncements	Revenue Recognition In May 2014, the Financial Accounting Standards Board (“FASB”) updated the accounting guidance related to revenue recognition. The updated accounting guidance provides a single, contract-based revenue recognition model to help improve financial reporting by providing clearer guidance on when an entity should recognize revenue and by reducing the number of standards to which an entity has to refer. The updated accounting guidance is effective for us as of January 1, 2018. We have substantially completed the review of our revenue arrangements and do not currently expect that the adoption of the new standard will have a material impact on our financial position or results of operations. However, we do expect that the new standard will impact the timing of recognition for (1) our Cable Communications segment’s installation revenue and commission expenses, which will be recognized as revenue and costs over a period of time instead of immediately, and (2) our Cable Networks, Broadcast Television and Filmed Entertainment segments’ content licensing revenue associated with renewals or extensions of existing program licensing agreements, which will be recognized as revenue when the licensed content becomes available under the renewal or extension instead of when the agreement is renewed or extended. In addition, upon adoption, we anticipate implementing certain changes in the presentation of revenue and expenses, including changes related to the allocation of revenue among the cable services included in a bundle that our residential customers purchase at a discount. The updated guidance also requires additional disclosures regarding the nature, timing and uncertainty of our revenue transactions. We intend to adopt the provisions of the guidance using the full retrospective method, under which we will adjust any prior periods presented to reflect the updated guidance. Financial Assets and Financial Liabilities In January 2016, the FASB updated the accounting guidance related to the recognition and measurement of financial assets and financial liabilities. The updated accounting guidance, among other things, requires that all nonconsolidated equity investments, except those accounted for under the equity method, be measured at fair value and that the changes in fair value be recognized in net income. The updated guidance is effective for us as of January 1, 2018. The updated accounting guidance requires a cumulative effect adjustment to beginning retained earnings in the year the guidance is adopted with certain exceptions. If we had adopted the provisions of the updated guidance as of January 1, 2017 for our equity investments classified as available-for-sale securities, primarily our investment in Snap Inc. (see Note 6), net income attributable to Comcast Corporation would have decreased for the three months ended June 30, 2017 by $88 million and increased for the six months ended June 30, 2017 by $16 million. We are currently in the process of determining the impact that the updated accounting guidance will have on our cost method investments. Leases In February 2016, the FASB updated the accounting guidance related to leases. The updated accounting guidance requires lessees to recognize a right-of-use asset and a lease liability on the balance sheet for all leases with the exception of short-term leases. The asset and liability are initially measured based on the present value of committed lease payments. For a lessee, the recognition, measurement and presentation of expenses and cash flows arising from a lease do not significantly change from previous guidance. For a lessor, the accounting applied is also largely unchanged from previous guidance. The updated guidance is effective for us as of January 1, 2019 and early adoption is permitted. The updated accounting guidance must be adopted using a modified retrospective approach for leases that exist or are entered into after the beginning of the earliest comparative period in the financial statements. We are currently in the process of determining the impact that the updated accounting guidance will have on our consolidated financial statements. Share-Based Compensation In March 2016, the FASB updated the accounting guidance that affects several aspects of the accounting for share-based compensation. The most significant change for us relates to the presentation of the income and withholding tax consequences of share-based compensation in our consolidated financial statements. Among the changes, the updated guidance requires that the excess income tax benefits or deficiencies that arise when the tax consequences of share-based compensation differ from amounts previously recognized in the statement of income be recognized as income tax benefit or expense in the statement of income rather than as additional paid-in capital in the balance sheet. The guidance also states that excess income tax benefits should not be presented separately from other income taxes in the statement of cash flows and, thus, should be classified as an operating activity rather than a financing activity as they were under the prior guidance. In addition, the updated guidance requires that, when an employer withholds shares upon exercise of options or the vesting of restricted stock for the purpose of meeting withholding tax requirements, the cash paid for withholding taxes be classified as a financing activity and we include these amounts in the caption “repurchases of common stock under repurchase program and employee plans” in our consolidated statement of cash flows. We previously recorded these amounts as operating activities. We adopted the updated guidance as of January 1, 2017 and, as required, we prospectively adopted the provisions that relate to the recognition of the excess income tax benefits or deficiencies in our condensed consolidated statement of income. The excess tax benefits resulted in a decrease to income tax expense of $59 million and $198 million for the three and six months ended June 30, 2017, respectively. In addition, the excess tax benefits resulted in an increase to diluted earnings per common share attributable to Comcast Corporation shareholders of $0.01 and $0.03 for the three and six months ended June 30, 2017, respectively. As required by the updated guidance, the prior year periods in our condensed consolidated statement of income were not adjusted as a result of these provisions. In addition, we retrospectively adopted the provisions of this guidance related to changes to the statement of cash flows for all periods presented. This resulted in increases to net cash provided by operating activities and decreases to net cash provided by (used in) financing activities of $543 million and $411 million for the six months ended June 30, 2017 and 2016, respectively.
NBCUniversal Media LLC [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Recent Accounting Pronouncements	Revenue Recognition In May 2014, the Financial Accounting Standards Board (“FASB”) updated the accounting guidance related to revenue recognition. The updated accounting guidance provides a single, contract-based revenue recognition model to help improve financial reporting by providing clearer guidance on when an entity should recognize revenue and by reducing the number of standards to which an entity has to refer. The updated accounting guidance is effective for us as of January 1, 2018. We have substantially completed the review of our revenue arrangements and do not currently expect that the adoption of the new standard will have a material impact on our financial position or results of operations. However, we do expect that the new standard will impact the timing of recognition for our Cable Networks, Broadcast Television and Filmed Entertainment segments’ content licensing revenue associated with renewals or extensions of existing program licensing agreements, which will be recognized as revenue when the licensed content becomes available under the renewal or extension instead of when the agreement is renewed or extended. The updated guidance also requires additional disclosures regarding the nature, timing and uncertainty of our revenue transactions. We intend to adopt the provisions of the guidance using the full retrospective method, under which we will adjust any prior periods presented to reflect the updated guidance. Financial Assets and Financial Liabilities In January 2016, the FASB updated the accounting guidance related to the recognition and measurement of financial assets and financial liabilities. The updated accounting guidance, among other things, requires that all nonconsolidated equity investments, except those accounted for under the equity method, be measured at fair value and that the changes in fair value be recognized in net income. The updated guidance is effective for us as of January 1, 2018. The updated accounting guidance requires a cumulative effect adjustment to beginning retained earnings in the year the guidance is adopted with certain exceptions. If we had adopted the provisions of the updated guidance as of January 1, 2017 for our equity investments classified as available-for-sale securities, primarily our investment in Snap Inc. (see Note 6), net income attributable to NBCUniversal would have decreased for the three and six months ended June 30, 2017 by $140 million and $139 million, respectively. We are currently in the process of determining the impact that the updated accounting guidance will have on our cost method investments. Leases In February 2016, the FASB updated the accounting guidance related to leases. The updated accounting guidance requires lessees to recognize a right-of-use asset and a lease liability on the balance sheet for all leases with the exception of short-term leases. The asset and liability are initially measured based on the present value of committed lease payments. For a lessee, the recognition, measurement and presentation of expenses and cash flows arising from a lease do not significantly change from previous guidance. For a lessor, the accounting applied is also largely unchanged from previous guidance. The updated guidance is effective for us as of January 1, 2019 and early adoption is permitted. The updated accounting guidance must be adopted using a modified retrospective approach for leases that exist or are entered into after the beginning of the earliest comparative period in the financial statements. We are currently in the process of determining the impact that the updated accounting guidance will have on our consolidated financial statements.