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Significant Accounting Policies - Additional information (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Depreciation Amortization Impairment [Line Items]
Operating lease right-of-use assets	$ 517
Operating lease liability	$ 538
Accounting Standards Update 2016-02 [Member]
Depreciation Amortization Impairment [Line Items]
Operating lease right-of-use assets		$ 553
Operating lease liability		$ 561