Acquisitions, Goodwill, and Acquired Intangible Assets - Allocation of Purchase Price (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Purchase Price
Cash paid, net of cash acquired	$ 633	$ 14	$ 678	$ 30	$ 765	$ 452
Allocation
Goodwill	4,254		4,254		4,254		$ 3,784
2017 Acquisitions
Purchase Price
Cash paid, net of cash acquired			586
Indemnification holdback			86
Purchase price			672
Allocation
Goodwill	425		425		425
Intangible assets	250		250		250
Property and equipment	14		14		14
Deferred tax assets	15		15		15
Other assets acquired	134		134		134
Deferred tax liabilities	(18)		(18)		(18)
Other liabilities assumed	(148)		(148)		(148)
Allocation	672		$ 672		672
Acquired intangible assets, weighted-average useful life			4 years
2017 Acquisitions | Minimum
Allocation
Acquired intangible assets, estimated useful life			1 year
2017 Acquisitions | Maximum
Allocation
Acquired intangible assets, estimated useful life			7 years
2017 Acquisitions | Marketing-related
Allocation
Intangible assets	62		$ 62		62
2017 Acquisitions | Contract-based
Allocation
Intangible assets	32		32		32
2017 Acquisitions | Technology-based
Allocation
Intangible assets	108		108		108
2017 Acquisitions | Customer-related
Allocation
Intangible assets	$ 48		$ 48		$ 48