Capital Structure - Outstanding preferred stock (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)	72,200	72,200
Aggregate liquidation preference	$ 1,805.0
Aggregate dividend payment ($ in millions)	$ 116.0	$ 116.0	$ 116.0
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 ($ in millions)	$ 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 ($ in millions)	$ 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	$ 1.66
Aggregate dividend payment ($ in millions)	$ 9.0	$ 9.0	$ 9.0
Series E
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)	29,900
Aggregate liquidation preference	$ 747.5
Dividend rate	6.625%
Dividends payable, amount per share (in usd per share)	$ 1.66	$ 1.66	$ 1.66
Aggregate dividend payment ($ in millions)	$ 49.0	$ 49.0	$ 49.0
Series F
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)	10,000
Aggregate liquidation preference	$ 250.0
Dividend rate	6.25%
Dividends payable, amount per share (in usd per share)	$ 1.56	$ 1.56	$ 1.56
Aggregate dividend payment ($ in millions)	$ 16.0	$ 16.0	$ 16.0