Deferred Revenue - Schedule of Rollforward (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Disaggregation Of Revenue [Roll Forward]
Beginning balance	$ 216	$ 194
Additions	652	648
Amortization	404	402
Divestitures		(10)
Ending balance	464	430
Annual Listings Revenues
Disaggregation Of Revenue [Roll Forward]
Beginning balance	0	0
Additions	383	366
Amortization	191	184
Divestitures		0
Ending balance	192	182
Original Listings Revenues
Disaggregation Of Revenue [Roll Forward]
Beginning balance	25	23
Additions	13	11
Amortization	12	11
Divestitures		0
Ending balance	26	23
Other Listings Revenues
Disaggregation Of Revenue [Roll Forward]
Beginning balance	98	83
Additions	26	39
Amortization	17	21
Divestitures		0
Ending balance	107	101
Data Services and Other Revenues
Disaggregation Of Revenue [Roll Forward]
Beginning balance	93	88
Additions	230	232
Amortization	184	186
Divestitures		(10)
Ending balance	$ 139	$ 124