Shareholders' Equity - Preferred Stock (Details)	Apr. 29, 2019 USD ($) $ / shares	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Class of Stock [Line Items]
Preferred stock issued		$ 1,450,000,000	$ 1,450,000,000
Subsequent Event [Member] | Series G Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock issued	$ 450,000,000
Preferred stock, par value (in usd per share) | $ / shares	$ 1
Preferred stock, liquidation preference, value	$ 1,000
Subsequent Event [Member] | Depository Shares, Series G [Member]
Class of Stock [Line Items]
Depository receipt ratio	0.025
Preferred stock, liquidation preference (in usd per share) | $ / shares	$ 25