Shareholders' Equity - Narrative (Details)		3 Months Ended					12 Months Ended
	Feb. 15, 2017 USD ($) $ / shares	Jun. 30, 2018 $ / shares	Dec. 31, 2017 USD ($) $ / shares shares	Sep. 30, 2017 $ / shares	Jun. 30, 2017 $ / shares shares	Mar. 31, 2017 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares	Mar. 20, 2017 shares	Dec. 12, 2016	Sep. 09, 2016
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Cash dividends declared per common share (in dollars per share)			$ 0.105	$ 0.095	$ 0.095	$ 0.085	$ 0.38	$ 0.33	$ 0.290
Common stock issued upon conversion (in shares) | shares										7.0922
Preferred stock | $			$ 1,025,000,000				$ 1,025,000,000	$ 1,665,000,000
Dividends payable | $							(480,000,000)	$ (335,000,000)	$ (267,000,000)
Series A Preferred Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock, dividend rate, percentage								7.75%
Preferred stock | $								$ 290,000,000
Preferred stock, par value (in dollars per share)								$ 1
Preferred stock, liquidation preference (in dollars per share)								$ 100
Preferred stock, shares authorized (in shares) | shares								7,475,000
Preferred stock, shares outstanding (in shares) | shares								2,900,234
Dividend payable (in dollars per share)						$ 1.9375
Dividends payable | $						$ (6,000,000)
Series C Preferred Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock | $								$ 350,000,000
Preferred stock, par value (in dollars per share)								$ 1
Preferred stock, liquidation preference (in dollars per share)	$ 25
Preferred stock, shares authorized (in shares) | shares								14,000,000
Dividend payable (in dollars per share)	$ 0.539063
Dividends payable | $	$ (7,000,000)
Depository Shares [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Shares issued | $							$ 525,000,000
Series D Preferred Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock, par value (in dollars per share)			$ 1				$ 1
Preferred stock, liquidation preference (in dollars per share)			$ 25,000				$ 25,000
Preferred stock, shares authorized (in shares) | shares			21,000				21,000
Preferred stock, shares outstanding (in shares) | shares			21,000				21,000
Dividend payable (in dollars per share)			$ 50				$ 50
Dividends payable | $			$ (26,000,000)
Depository receipt ratio												0.04
Depository Shares, Series E [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Shares issued | $							$ 500,000,000
Series E Preferred Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock, par value (in dollars per share)			$ 1				$ 1
Preferred stock, liquidation preference (in dollars per share)			$ 1,000				$ 1,000
Preferred stock, shares authorized (in shares) | shares			500,000				500,000
Preferred stock, shares outstanding (in shares) | shares					500,000
Dividend payable (in dollars per share)			$ 1.544012				$ 1.544012
Dividends payable | $			$ (31,000,000)
Depository receipt ratio											0.025
2017 Capital Plan [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Stock repurchase program authorized amount (in shares) | $			$ 800,000,000				$ 800,000,000
Repurchase of common stock | $							476,000,000
2016 Capital Plan [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Repurchase of common stock | $							$ 254,000,000
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 (in dollars per share)		$ 0.120