Shareholders' Equity - Preferred Stock (Details)	Sep. 30, 2018 USD ($) $ / shares	Jul. 30, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($)
Class of Stock [Line Items]
Preferred stock issued	$ 1,450,000,000		$ 1,025,000,000
Series F Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock issued		$ 425,000,000
Preferred stock, par value (in usd per share) | $ / shares	$ 1	$ 1
Preferred stock, liquidation preference, value	$ 1,000	$ 1,000
Depository Shares, Series F [Member]
Class of Stock [Line Items]
Depository receipt ratio	25,000.000	0.025
Preferred stock, liquidation preference (in usd per share) | $ / shares	$ 25	$ 25