Shareholders' Equity - Summary of Preferred Stock (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Carrying Amount	$ 3,053	$ 3,053
Liquidation Amount	3,140
Series D
Class of Stock [Line Items]
Carrying Amount	559
Liquidation Amount	$ 575
Dividend Rate	5.85%
Series E
Class of Stock [Line Items]
Carrying Amount	$ 1,120
Liquidation Amount	$ 1,150
Dividend Rate	5.625%
Series F
Class of Stock [Line Items]
Carrying Amount	$ 437
Liquidation Amount	$ 450
Dividend Rate	5.20%
Series G
Class of Stock [Line Items]
Carrying Amount	$ 487
Liquidation Amount	$ 500
Dividend Rate	5.20%
Series H
Class of Stock [Line Items]
Carrying Amount	$ 450
Liquidation Amount	$ 465
Dividend Rate	5.625%