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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

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