Capital Structure Capital Structure, Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		12 Months Ended
	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Class of Stock [Line Items]
Preferred stock, shares issued			72,200	72,200
Aggregate liquidation preference			$ 1,805.0
Dividends Payable, Amount Per Share
Preferred stock dividends			$ 116.0	$ 104.0	$ 17.0
Series A
Class of Stock [Line Items]
Preferred stock, shares issued			11,500
Aggregate liquidation preference			$ 287.5
Dividend rate			5.625%
Dividends Payable, Amount Per Share			$ 1.41	$ 1.41	$ 0.83
Preferred stock dividends			$ 16.0	$ 16.0	$ 9.0
Series C
Class of Stock [Line Items]
Preferred stock, shares issued			15,400
Aggregate liquidation preference			$ 385.0
Dividend rate			6.75%
Dividends Payable, Amount Per Share			$ 1.69	$ 1.69	$ 0.49
Preferred stock dividends			$ 26.0	$ 26.0	$ 8.0
Series D
Class of Stock [Line Items]
Preferred stock, shares issued			5,400
Aggregate liquidation preference			$ 135.0
Dividend rate			6.625%
Dividends Payable, Amount Per Share			$ 1.66	$ 1.79	$ 0.00
Preferred stock dividends			$ 9.0	$ 10.0	$ 0.0
Series E
Class of Stock [Line Items]
Preferred stock, shares issued		29,900	29,900
Aggregate liquidation preference		$ 747.5	$ 747.5
Dividend rate			6.625%
Dividends Payable, Amount Per Share			$ 1.66	$ 1.44	$ 0.00
Preferred stock dividends			$ 49.0	$ 43.0	$ 0.0
Series F
Class of Stock [Line Items]
Preferred stock, shares issued	10,000		10,000
Aggregate liquidation preference	$ 250.0		$ 250.0
Dividend rate			6.25%
Dividends Payable, Amount Per Share			$ 1.56	$ 0.92	$ 0.00
Preferred stock dividends			$ 16.0	$ 9.0	$ 0.0