Stockholders' Equity - Preferred Stock Outstanding (Details) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($)
Class of Stock [Line Items]
Preferred Stock, Including Additional Paid in Capital, Net of Discount	$ 5,330	$ 4,853
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 as of September 30, 2020 | shares	0
Preferred Stock, Including Additional Paid in Capital, Net of Discount	$ 0	853
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 as of September 30, 2020 | shares	1,000,000
Preferred Stock, Including Additional Paid in Capital, Net of Discount	$ 988	988
Preferred Stock Dividend Rate, Variable	3.80%
Preferred Stock, Dividend Payment Rate, Variable	LIBOR + 380 bps
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 as of September 30, 2020 | 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 as of September 30, 2020 | 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 as of September 30, 2020 | shares	500,000
Preferred Stock, Including Additional Paid in Capital, Net of Discount	$ 483	483
Series I Preferred Stock
Class of Stock [Line Items]
Per Annum Dividend Rate	5.00%
Liquidation Preference per Share | $ / shares	$ 1,000
Total Shares Outstanding as of September 30, 2020 | shares	1,500,000
Preferred Stock, Including Additional Paid in Capital, Net of Discount	$ 1,462	1,462
Series J Preferred Stock
Class of Stock [Line Items]
Per Annum Dividend Rate	4.80%
Liquidation Preference per Share | $ / shares	$ 1,000
Total Shares Outstanding as of September 30, 2020 | shares	1,250,000
Preferred Stock, Including Additional Paid in Capital, Net of Discount	$ 1,209	0
Series K Preferred Stock
Class of Stock [Line Items]
Per Annum Dividend Rate	4.625%
Liquidation Preference per Share | $ / shares	$ 1,000
Total Shares Outstanding as of September 30, 2020 | shares	125,000
Preferred Stock, Including Additional Paid in Capital, Net of Discount	$ 121	$ 0