Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of intangible assets	$ 33	$ 4	$ 6
2048 Senior Notes (4.25% senior unsecured notes due September 21, 2048)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes		1,230	0
Fair value disclosure		1,240
2028 Senior Notes (3.75% senior unsecured notes due September 21, 2028)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes		591	0
Fair value disclosure		599
2027 Senior Notes (3.10% senior unsecured notes due September 15, 2027)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes		496	495
Fair value disclosure		477
2025 Senior Notes (3.75% senior unsecured notes due December 1, 2025)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes		1,240	1,242
Fair value disclosure		1,260
2023 Senior Notes (4.00% senior unsecured notes due October 15, 2023)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes		793	791
Fair value disclosure		821
2023 Senior Notes (3.45% senior unsecured notes due September 21, 2023)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes		397	0
Fair value disclosure		402
2022 Senior Notes (2.35% senior unsecured notes due September 15, 2022)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes		496	495
Fair value disclosure		484
2020 Senior Notes (2.75% senior unsecured notes due December 1, 2020)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes		1,250	$ 1,244
Fair value disclosure		$ 1,240