Employee Benefit Plans (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash surrender value of life insurance policies	$ 820	$ 709
Expenses related to retirement investment plans	458	446	$ 416
Severance Costs	203	315	181
NBCUniversal Media LLC [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expenses related to retirement investment plans	201	185	174
Severance Costs	108	165	113
DreamWorks Animation [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Severance Costs		61
DreamWorks Animation [Member] | NBCUniversal Media LLC [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Severance Costs		61
Multiemployer Plans, Pension [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Multiemployer plan contributions	97	84	77
Multiemployer Plans, Pension [Member] | NBCUniversal Media LLC [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Multiemployer plan contributions	97	84	77
Multiemployer Plans, Postretirement Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Multiemployer plan contributions	152	136	119
Multiemployer Plans, Postretirement Benefit [Member] | NBCUniversal Media LLC [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Multiemployer plan contributions	152	136	$ 119
Postretirement Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Reduction of benefit obligations due to plan amendments	146	361
Benefit obligation	425	538
Postretirement Benefit Plan [Member] | NBCUniversal Media LLC [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Reduction of benefit obligations due to plan amendments	146
Benefit obligation	51	192
Pension Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Benefit obligation	338	314
Pension Plans [Member] | NBCUniversal Media LLC [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Benefit obligation	$ 338	$ 314