Summary of effects of initial adoption of ASU 2016-01, ASU 2018-02 and ASC 606 (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated other comprehensive income	$ (4,243)	$ (5,243)
Retained earnings	444,626	402,493
Total shareholders’ equity	$ 451,336	$ 428,563	$ 352,500		$ 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)