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Summary of effects of initial adoption of ASU 2016-01, ASU 2018-02 and ASC 606 (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Assets	$ 736,459		$ 702,095
Liabilities	357,040		350,141
Accumulated other comprehensive income	(3,927)		58,571
Retained earnings	346,503		255,786
Total shareholders' equity	$ 379,419		$ 351,954	$ 310,921	$ 285,428
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		(71)
Total shareholders' equity		(71)
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		61,304
Total shareholders' equity		$ (71)