Stockholders' Equity - Schedule of Preferred Stock (Details) $ / shares in Units, $ in Millions	6 Months Ended
	Jun. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($)
Class of Stock [Line Items]
Preferred Stock, Including Additional Paid in Capital, Net of Discount	$ 4,360	$ 4,360
Depository Share, Percent Interest in Preferred Stock	0.025
Series B Preferred Stock
Class of Stock [Line Items]
Per Annum Dividend Rate	6.00%
Liquidation Preference per Share | $ / shares	$ 1,000
Total Shares Outstanding | shares	875,000
Preferred Stock, Including Additional Paid in Capital, Net of Discount	$ 853	853
Series C Preferred Stock
Class of Stock [Line Items]
Per Annum Dividend Rate	6.25%
Liquidation Preference per Share | $ / shares	$ 1,000
Total Shares Outstanding | shares	500,000
Preferred Stock, Including Additional Paid in Capital, Net of Discount	$ 484	484
Series D Preferred Stock
Class of Stock [Line Items]
Per Annum Dividend Rate	6.70%
Liquidation Preference per Share | $ / shares	$ 1,000
Total Shares Outstanding | shares	500,000
Preferred Stock, Including Additional Paid in Capital, Net of Discount	$ 485	485
Series E Preferred Stock
Class of Stock [Line Items]
Per Annum Dividend Rate	5.55%
Liquidation Preference per Share | $ / shares	$ 1,000
Total Shares Outstanding | shares	1,000,000
Preferred Stock, Including Additional Paid in Capital, Net of Discount	$ 988	988
Preferred Stock, Dividend Payment Rate, Variable	Libor + 380 bps
Preferred Stock Dividend Rate, Variable	3.80%
Series F Preferred Stock
Class of Stock [Line Items]
Per Annum Dividend Rate	6.20%
Liquidation Preference per Share | $ / shares	$ 1,000
Total Shares Outstanding | shares	500,000
Preferred Stock, Including Additional Paid in Capital, Net of Discount	$ 484	484
Series G Preferred Stock
Class of Stock [Line Items]
Per Annum Dividend Rate	5.20%
Liquidation Preference per Share | $ / shares	$ 1,000
Total Shares Outstanding | shares	600,000
Preferred Stock, Including Additional Paid in Capital, Net of Discount	$ 583	583
Series H Preferred Stock
Class of Stock [Line Items]
Per Annum Dividend Rate	6.00%
Liquidation Preference per Share | $ / shares	$ 1,000
Total Shares Outstanding | shares	500,000
Preferred Stock, Including Additional Paid in Capital, Net of Discount	$ 483	$ 483