Segments - Business Segment Information (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenues		$ 1,600	$ 1,641	$ 4,857	$ 4,811
Depreciation and amortization		175	160	504	477
Restructuring charges		19	7	108	48
Stock-based compensation expense		1	8	21	35
Other items	[1]	15	10	33	28
Operating income/(loss)		261	334	696	860
Business segment income/(loss)	[2]	471	519	1,362	1,448
Total assets		16,635		16,635		$ 16,866
Operating Segments | Buy
Segment Reporting Information [Line Items]
Revenues		755	803	2,320	2,383
Depreciation and amortization		58	53	167	156
Restructuring charges		16	4	86	31
Stock-based compensation expense		2	3	9	10
Operating income/(loss)		33	84	41	217
Business segment income/(loss)	[2]	109	144	303	414
Total assets		6,877		6,877		6,862
Operating Segments | Watch
Segment Reporting Information [Line Items]
Revenues		845	838	2,537	2,428
Depreciation and amortization		115	106	332	318
Restructuring charges		4	2	18	9
Stock-based compensation expense		2	2	7	9
Operating income/(loss)		256	280	738	734
Business segment income/(loss)	[2]	377	390	1,095	1,070
Total assets		9,768		9,768		9,911
Corporate And Eliminations
Segment Reporting Information [Line Items]
Depreciation and amortization		2	1	5	3
Restructuring charges		(1)	1	4	8
Stock-based compensation expense		(3)	3	5	16
Other items	[1]	15	10	33	28
Operating income/(loss)		(28)	(30)	(83)	(91)
Business segment income/(loss)	[2]	(15)	(15)	(36)	(36)
Total assets		(10)		(10)		$ 93
Corporate And Eliminations | Buy
Segment Reporting Information [Line Items]
Revenues		5	15	24	51
Corporate And Eliminations | Watch
Segment Reporting Information [Line Items]
Revenues		$ 33	$ 42	$ 103	$ 139

[1] Other items primarily consist of business optimization costs and transaction related costs for the three and nine months ended September 30, 2018 and 2017.

[2] The Company’s chief operating decision maker uses business segment income/(loss) to measure performance from period to period both at the consolidated level as well as within its operating segments.