Separation Transaction and Discontinued Operations - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended			3 Months Ended		9 Months Ended
	Nov. 01, 2016	May 31, 2017	Apr. 30, 2017	Feb. 28, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Common stock sold				23,353,000	12,958,767
Common stock sold per share				$ 38.03	$ 35.91
Cash proceeds sale of common stock				$ 888
Pre-tax gain on sale of common stock				$ 351			$ 167
Investment in common stock of Alcoa Corporation							0	$ 1,020
Investment Banks [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Common stock sold		12,958,767	12,958,767
Common stock sold per share		$ 35.91	$ 35.91
Pre-tax gain on sale of common stock		$ 167	$ 167
Yadkin [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of assets					$ 5	$ 238	$ 243
Alcoa Corporation [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Common stock outstanding percentage	80.10%
Number of common stock retained							36,311,767
Percentage of common stock retained							19.90%