Acquisitions, Goodwill, and Acquired Intangible Assets - Allocation of Aggregate Purchase Price of Acquisitions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Purchase Price
Cash paid, net of cash acquired	$ 2,186	$ 13,972	$ 116
Allocation
Goodwill	$ 14,548	13,350	3,784
Acquired intangibles weighted average amortization period	15 years 5 months
Minimum
Allocation
Intangible assets, estimated useful life	1 year
Maximum
Allocation
Intangible assets, estimated useful life	25 years
Marketing-related
Allocation
Acquired intangibles weighted average amortization period	21 years 2 months
Contract-based
Allocation
Acquired intangibles weighted average amortization period	12 years 4 months
Technology-based
Allocation
Acquired intangibles weighted average amortization period	4 years 7 months
Customer-related
Allocation
Acquired intangibles weighted average amortization period	4 years 5 months
2016 Acquisitions
Purchase Price
Cash paid, net of cash acquired			81
Indemnification holdback			22
Aggregate purchase price			103
Allocation
Goodwill			60
Intangible assets			57
Property and equipment			3
Deferred tax assets			17
Other assets acquired			10
Long-term debt			(5)
Deferred tax liabilities			(18)
Other liabilities assumed			(21)
Allocated purchase price			$ 103
Acquired intangibles weighted average amortization period			5 years
2016 Acquisitions | Minimum
Allocation
Intangible assets, estimated useful life			1 year
2016 Acquisitions | Maximum
Allocation
Intangible assets, estimated useful life			7 years
2016 Acquisitions | Marketing-related
Allocation
Intangible assets			$ 2
2016 Acquisitions | Contract-based
Allocation
Intangible assets			1
2016 Acquisitions | Technology-based
Allocation
Intangible assets			53
2016 Acquisitions | Customer-related
Allocation
Intangible assets			$ 1
2017 Acquisitions
Purchase Price
Cash paid, net of cash acquired		13,859
Indemnification holdback		104
Aggregate purchase price		13,963
Allocation
Goodwill		9,501
Intangible assets		2,647
Property and equipment		3,810
Deferred tax assets		117
Other assets acquired		1,858
Long-term debt		(1,165)
Deferred tax liabilities		(961)
Other liabilities assumed		(1,844)
Allocated purchase price		$ 13,963
Acquired intangibles weighted average amortization period		21 years
2017 Acquisitions | Minimum
Allocation
Intangible assets, estimated useful life		1 year
2017 Acquisitions | Maximum
Allocation
Intangible assets, estimated useful life		25 years
2017 Acquisitions | Marketing-related
Allocation
Intangible assets		$ 1,987
2017 Acquisitions | Contract-based
Allocation
Intangible assets		440
2017 Acquisitions | Technology-based
Allocation
Intangible assets		166
2017 Acquisitions | Customer-related
Allocation
Intangible assets		$ 54
2018 Acquisitions
Purchase Price
Cash paid, net of cash acquired	$ 1,618
Indemnification holdback	31
Aggregate purchase price	1,649
Allocation
Goodwill	1,228
Intangible assets	677
Property and equipment	11
Deferred tax assets	174
Other assets acquired	282
Long-term debt	(176)
Deferred tax liabilities	(159)
Other liabilities assumed	(388)
Allocated purchase price	$ 1,649
Acquired intangibles weighted average amortization period	6 years
2018 Acquisitions | Minimum
Allocation
Intangible assets, estimated useful life	2 years
2018 Acquisitions | Maximum
Allocation
Intangible assets, estimated useful life	7 years
2018 Acquisitions | Marketing-related
Allocation
Intangible assets	$ 186
2018 Acquisitions | Contract-based
Allocation
Intangible assets	13
2018 Acquisitions | Technology-based
Allocation
Intangible assets	285
2018 Acquisitions | Customer-related
Allocation
Intangible assets	$ 193