Summary of effects of initial adoption of ASU 2016-01, ASU 2018-02 and ASC 606 (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated other comprehensive income	$ (5,408)			$ (5,015)
Retained earnings	373,334			321,112
Total shareholders’ equity	$ 401,558	$ 386,391	$ 372,591	$ 352,500	$ 379,419	$ 361,758	$ 351,052		$ 351,954
ASU 2016-01 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated other comprehensive income								$ (61,459)
Retained earnings								61,459
ASU 2018-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated other comprehensive income								84
Retained earnings								(84)
ASC 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings								(70)
Total shareholders’ equity								(70)
Adjustments for New Accounting Pronouncements [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated other comprehensive income								(61,375)
Retained earnings								61,305
Total shareholders’ equity								$ (70)