Shareholders' Equity - Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended				6 Months Ended
	Jul. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Class of Stock [Line Items]
Preferred stock cash dividend		$ 50	$ 55	$ 36	$ 55	$ 105	$ 91
Series C Preferred Stock
Class of Stock [Line Items]
Preferred dividends declared (USD per share)		$ 1,313		$ 1,313		$ 2,626	$ 2,626
Preferred stock cash dividend		$ 7		$ 7		$ 13	$ 13
Series C Preferred Stock, Depository Share
Class of Stock [Line Items]
Preferred dividends declared (USD per share)		$ 0.33		$ 0.33		$ 0.66	$ 0.66
Series D Preferred Stock
Class of Stock [Line Items]
Preferred dividends declared (USD per share)		$ 1,475		$ 1,475		$ 2,950	$ 2,950
Preferred stock cash dividend		$ 11		$ 11		$ 22	$ 22
Series D Preferred Stock, Depository Share
Class of Stock [Line Items]
Preferred dividends declared (USD per share)		$ 0.37		$ 0.37		$ 0.74	$ 0.74
Series E Preferred Stock
Class of Stock [Line Items]
Preferred dividends declared (USD per share)		$ 1,500		$ 1,500		$ 3,000	$ 3,000
Preferred stock cash dividend		$ 11		$ 11		$ 22	$ 22
Series E Preferred Stock, Depository Share
Class of Stock [Line Items]
Preferred dividends declared (USD per share)		$ 0.38		$ 0.38		$ 0.76	$ 0.76
Series F Preferred Stock
Class of Stock [Line Items]
Preferred dividends declared (USD per share)		$ 0		$ 0		$ 2,625	$ 2,625
Preferred stock cash dividend		$ 0		$ 0		$ 20	$ 20
Series F Preferred Stock, Depository Share
Class of Stock [Line Items]
Preferred dividends declared (USD per share)		$ 0		$ 0		$ 26.25	$ 26.25
Series G Preferred Stock
Class of Stock [Line Items]
Preferred dividends declared (USD per share)		$ 1,338		$ 1,338		$ 2,676	$ 2,676
Preferred stock cash dividend		$ 7		$ 7		$ 14	$ 14
Series G Preferred Stock, Depository Share
Class of Stock [Line Items]
Preferred dividends declared (USD per share)		$ 0.33		$ 0.33		$ 0.66	$ 0.66
Subsequent Event | Series C Preferred Stock
Class of Stock [Line Items]
Preferred dividends declared (USD per share)	$ 1,313
Preferred stock cash dividend	$ 6
Subsequent Event | Series C Preferred Stock, Depository Share
Class of Stock [Line Items]
Preferred dividends declared (USD per share)	$ 0.33
Subsequent Event | Series D Preferred Stock
Class of Stock [Line Items]
Preferred dividends declared (USD per share)	$ 1,475
Preferred stock cash dividend	$ 11
Subsequent Event | Series D Preferred Stock, Depository Share
Class of Stock [Line Items]
Preferred dividends declared (USD per share)	$ 0.37
Subsequent Event | Series E Preferred Stock
Class of Stock [Line Items]
Preferred dividends declared (USD per share)	$ 1,500
Preferred stock cash dividend	$ 11
Subsequent Event | Series E Preferred Stock, Depository Share
Class of Stock [Line Items]
Preferred dividends declared (USD per share)	$ 0.38
Subsequent Event | Series F Preferred Stock
Class of Stock [Line Items]
Preferred dividends declared (USD per share)	$ 2,625
Preferred stock cash dividend	$ 20
Subsequent Event | Series F Preferred Stock, Depository Share
Class of Stock [Line Items]
Preferred dividends declared (USD per share)	$ 26.25
Subsequent Event | Series G Preferred Stock
Class of Stock [Line Items]
Preferred dividends declared (USD per share)	$ 1,338
Preferred stock cash dividend	$ 7
Subsequent Event | Series G Preferred Stock, Depository Share
Class of Stock [Line Items]
Preferred dividends declared (USD per share)	$ 0.33