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Fair Value of Derivatives and Financial Instruments Fair Value, by Balance Sheet Grouping (Details) - USD ($) $ / shares in Units, $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Preferred Stock, Redemption Date | Oct. 01, 2026 |
Preferred Stock, Redemption Price Per Share | $ 50.00 |
Fixed Rate Debt |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Notes Payable Excluding Interest Rate Swaps and Cap Agreements | $ 3,200.0 | $ 3,000.0
Notes Payable, Fair Value Disclosure | 3,300.0 | 3,100.0
Floating Rate Debt |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Notes Payable Excluding Interest Rate Swaps and Cap Agreements | 1,300.0 | 1,500.0
Notes Payable, Fair Value Disclosure | 1,300.0 | 1,500.0
Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Interest Rate Derivative Assets, at Fair Value | 3.6 | 2.4
Interest Rate Derivative Liabilities, at Fair Value | 1.3 | 7.6
Embedded Derivative, Fair Value of Embedded Derivative Asset | 21.2 | $ 10.8
Embedded Derivative, Gain on Embedded Derivative | 10.4 |
Business Combinations, Purchase Price Adjustment, Subsequent Adjustment | 1.6 |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Change in Unrealized Gain (Loss) | $ 8.8 |

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