Stockholders' Equity (Dividends Declared) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Class of Stock [Line Items]
Dividends declared	$ 79.2	$ 34.0	$ 159.9	$ 114.7
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared	$ 0.0	$ 0.0	$ 14.0	$ 14.0
Dividends declared, per share amount (USD per share)	$ 0	$ 0	$ 35.00	$ 35.00
Series C Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared	$ 9.0	$ 9.0	$ 27.0	$ 27.0
Dividends declared, per share amount (USD per share)	$ 15.00	$ 15.00	$ 45.00	$ 45.00
Series D Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared	$ 11.2	$ 11.2	$ 33.5	$ 33.5
Dividends declared, per share amount (USD per share)	$ 14.88	$ 14.88	$ 44.64	$ 44.64
Series E Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared	$ 13.8	$ 13.8	$ 27.7	$ 27.7
Dividends declared, per share amount (USD per share)	$ 2,312.50	$ 2,312.50	$ 4,625.00	$ 4,625.00
Series F Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared	$ 0.0	$ 0.0	$ 12.5	$ 12.5
Dividends declared, per share amount (USD per share)	$ 0	$ 0	$ 2,500.00	$ 2,500.00
Series G Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared	$ 45.2	$ 0.0	$ 45.2	$ 0.0
Dividends declared, per share amount (USD per share)	$ 1,806.60	$ 0	$ 1,806.60	$ 0