Stockholders' Equity (Dividends Declared) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Class of Stock [Line Items]
Dividends declared	$ 46.6	$ 46.6	$ 80.7	$ 80.7
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared	$ 14.0	$ 14.0	$ 14.0	$ 14.0
Dividends declared, per share amount (USD per share)	$ 35.00	$ 35.00	$ 35.00	$ 35.00
Series C Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared	$ 9.0	$ 9.0	$ 18.0	$ 18.0
Dividends declared, per share amount (USD per share)	$ 15.00	$ 15.00	$ 30.00	$ 30.00
Series D Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared	$ 11.1	$ 11.1	$ 22.3	$ 22.3
Dividends declared, per share amount (USD per share)	$ 14.88	$ 14.88	$ 29.76	$ 29.76
Series E Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared	$ 0.0	$ 0.0	$ 13.9	$ 13.9
Dividends declared, per share amount (USD per share)	$ 0	$ 0	$ 2,312.50	$ 2,312.50
Series F Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared	$ 12.5	$ 12.5	$ 12.5	$ 12.5
Dividends declared, per share amount (USD per share)	$ 2,500.00	$ 2,500.00	$ 2,500.00	$ 2,500.00
Series G Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Dividends declared, per share amount (USD per share)	$ 0	$ 0	$ 0	$ 0