Earnings per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net income (loss)		$ (11.2)	$ (155.1)	$ 148.6	$ (195.0)
Less: Net income attributable to noncontrolling interests		12.5	12.5	40.4	34.3
Less: Dividends on preferred stock		30.3	29.4	90.3	87.8
Net income (loss) attributable to common shareholders		$ (54.0)	$ (197.0)	$ 17.9	$ (317.1)
Weighted average shares outstanding - basic (in shares)		226.5	215.6	222.1	203.8
Net income available per common share - basic (in dollars per share)		$ (0.24)	$ (0.91)	$ 0.08	$ (1.56)
Weighted average shares outstanding - basic (in shares)		226.5	215.6	222.1	203.8
Dilutive effect of common stock equivalents	[1]			1.7
Weighted average shares outstanding - diluted (in shares)		226.5	215.6	223.8	203.8
Net income available per common share - diluted (in dollars per share)		$ (0.24)	$ (0.91)	$ 0.08	$ (1.56)

[1] The dilutive effects of common stock equivalents were computed using the treasury method for warrants and unvested stock awards, and the if-converted method for convertible preferred stock. For the periods with net income attributable to common shareholders, the anti-dilution sequencing rule was applied from the most dilutive to the least dilutive potential common shares.