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Summary of effects of initial adoption of ASU 2016-01, ASU 2018-02 and ASC 606 (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Assets	$ 707,794	$ 702,095	$ 620,854
Liabilities	355,294	350,141
Accumulated other comprehensive income	(5,015)	58,571
Retained earnings	321,112	255,786
Total shareholders' equity	352,500	$ 351,954	$ 285,428	$ 257,696
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]
Assets	3,382
Liabilities	3,453
Retained earnings	(70)
Total shareholders' equity	(70)
ASU 2016-01 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated other comprehensive income	(61,459)
Retained earnings	61,459
Adjustments for New Accounting Pronouncements [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Assets	3,382
Liabilities	3,453
Accumulated other comprehensive income	(61,375)
Retained earnings	61,305
Total shareholders' equity	$ (70)