Stockholders' Equity (Dividends Declared) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Class of Stock [Line Items]
Dividends declared	$ 34.1	$ 34.1
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared	$ 0.0	$ 0.0
Dividends declared, per share amount (USD per share)	$ 0	$ 0
Series C Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared	$ 9.0	$ 9.0
Dividends declared, per share amount (USD per share)	$ 15.00	$ 15.00
Series D Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared	$ 11.2	$ 11.2
Dividends declared, per share amount (USD per share)	$ 14.88	$ 14.88
Series E Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared	$ 13.9	$ 13.9
Dividends declared, per share amount (USD per share)	$ 2,312.50	$ 2,312.50
Series F Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared	$ 0.0	$ 0.0
Dividends declared, per share amount (USD per share)	$ 0	$ 0