Summary of Significant Accounting Policies - New Accounting Pronouncement (Details) - USD ($) $ in Millions		3 Months Ended
	Jan. 01, 2019	Mar. 31, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Lease liabilities		$ 1,220
Stranded tax effects reclassified		$ (84)
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Right-of-use assets	$ 910
Lease liabilities	1,060
Accounting Standards Update 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stranded tax effects reclassified	$ 84