Shareholders' Equity - Additional Information (Detail)		3 Months Ended				9 Months Ended		12 Months Ended
	Feb. 15, 2017 USD ($) $ / shares	Jun. 30, 2018 $ / shares	Dec. 31, 2017 $ / shares	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 $ / shares	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares shares	Jun. 30, 2017 $ / shares	Mar. 20, 2017 shares
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Repurchase of common shares | $						$ 466,000,000	$ 73,000,000
Cash dividends declared on common shares (in usd per share) | $ / shares				$ 0.095	$ 0.085	$ 0.275	$ 0.245
Common stock issued upon conversion (in shares) | shares										7.0922
Preferred stock | $				$ 1,025,000,000		$ 1,025,000,000		$ 1,665,000,000
Preferred stock, shares outstanding (in shares) | shares								2,900,234
Dividend payable | $						$ 353,000,000	$ 224,000,000
Series A Preferred Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock, dividend rate								7.75%
Dividend payable per share | $ / shares									$ 1.9375
Cash dividends declared on Noncumulative Series A Preferred Stock | $				$ 6,000,000
Preferred stock | $								$ 290,000,000
Preferred stock, par value (in usd per share) | $ / shares								$ 1
Preferred stock, liquidation preference | $ / shares								$ 100
Preferred stock, shares authorized (in shares) | shares								7,475,000
Series C Preferred Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Dividend payable per share | $ / shares	$ 0.539063
Preferred stock | $								$ 350,000,000
Preferred stock, par value (in usd per share) | $ / shares								$ 1
Preferred stock, liquidation preference | $ / shares								$ 25
Preferred stock, shares authorized (in shares) | shares								14,000,000
Preferred stock, shares outstanding (in shares) | shares				14,000,000		14,000,000		14,000,000
Preferred stock, redemption price per share (in usd per share) | $ / shares	$ 25
Dividend payable | $	$ 7,000,000
Depository Shares, Series D [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Shares issued | $						$ 525,000,000		$ 525,000,000
Series D Preferred Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Dividend payable per share | $ / shares				$ 12.50		$ 12.50
Preferred stock, par value (in usd per share) | $ / shares				1		1
Preferred stock, liquidation preference | $ / shares				$ 25,000		$ 25,000
Preferred stock, shares authorized (in shares) | shares				21,000		21,000		21,000
Preferred stock, shares outstanding (in shares) | shares				21,000		21,000		21,000
Dividend payable | $				$ 7,000,000
Depository receipt ratio				0.04		0.04		0.04
Depository Shares, Series E [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Shares issued | $						$ 500,000,000		$ 500,000,000
Series E Preferred Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Dividend payable per share | $ / shares				$ 0.382813		$ 0.382813
Preferred stock, par value (in usd per share) | $ / shares				1		1
Preferred stock, liquidation preference | $ / shares				$ 1,000		$ 1,000
Preferred stock, shares authorized (in shares) | shares				500,000		500,000		500,000
Preferred stock, shares outstanding (in shares) | shares				500,000		500,000		500,000
Dividend payable | $				$ 8,000,000
Depository receipt ratio				0.025		0.025		0.025
2017 Capital Plan [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Authorized amount of share repurchases | $				$ 800,000,000		$ 800,000,000
Common shares repurchased, value | $				277,000,000
Repurchase of common shares | $				271,000,000
Employee equity compensation program common share repurchases | $				$ 6,000,000
Cash dividends declared on common shares (in usd per share) | $ / shares				$ 0.095
Cash dividends declared on common shares | $				$ 102,000,000
Scenario, Forecast [Member] | 2017 Capital Plan [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Proposed increase in cash dividends declared on common shares, per share | $ / shares		$ 0.120	$ 0.105