Summary of Significant Accounting Policies - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Weighted average remaining lease term	5 years 9 months 18 days
Weighted average discount rate	3.00%
Operating lease costs	$ 74
Operating lease payments	$ 71
Minimum | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, term of contract	1 year
Maximum | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, term of contract	37 years