Capital Structure, Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		3 Months Ended		12 Months Ended
	Jun. 30, 2014	Mar. 31, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)					72,200	72,200
Aggregate liquidation preference					$ 1,805.0
Aggregate Dividend Payment					$ 116.0	$ 116.0	$ 104.0
Series A
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)					11,500
Aggregate liquidation preference					$ 287.5
Dividend rate					5.625%
Dividends payable, amount per share (in usd per share)					$ 1.41	$ 1.41	$ 1.41
Aggregate Dividend Payment					$ 16.0	$ 16.0	$ 16.0
Series C
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)					15,400
Aggregate liquidation preference					$ 385.0
Dividend rate					6.75%
Dividends payable, amount per share (in usd per share)					$ 1.69	$ 1.69	$ 1.69
Aggregate Dividend Payment					$ 26.0	$ 26.0	$ 26.0
Series D
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)					5,400
Aggregate liquidation preference					$ 135.0
Dividend rate					6.625%
Dividends payable, amount per share (in usd per share)					$ 1.66	$ 1.66	$ 1.79
Aggregate Dividend Payment					$ 9.0	$ 9.0	$ 10.0
Series E
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)		29,900		29,900	29,900
Aggregate liquidation preference		$ 747.5			$ 747.5
Dividend rate				6.625%	6.625%
Dividends payable, amount per share (in usd per share)					$ 1.66	$ 1.66	$ 1.44
Aggregate Dividend Payment					$ 49.0	$ 49.0	$ 43.0
Series F
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)	10,000		10,000		10,000
Aggregate liquidation preference	$ 250.0				$ 250.0
Dividend rate			6.25%		6.25%
Dividends payable, amount per share (in usd per share)					$ 1.56	$ 1.56	$ 0.92
Aggregate Dividend Payment					$ 16.0	$ 16.0	$ 9.0