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Financial Instruments and Derivatives Fair Value, by Balance Sheet Grouping (Details) - USD ($) $ / shares in Units, $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017
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 | $ 3,100.0 | $ 3,200.0
Notes Payable, Fair Value Disclosure | 3,100.0 | 3,300.0
Floating Rate Debt |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Notes Payable Excluding Interest Rate Swaps and Cap Agreements | 1,100.0 | 1,300.0
Notes Payable, Fair Value Disclosure | 1,100.0 | 1,300.0
Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Interest Rate Derivative Assets, at Fair Value | 3.7 | 3.6
Interest Rate Derivative Liabilities, at Fair Value | 5.3 | 1.3
Embedded Derivative, Fair Value of Embedded Derivative Asset | $ 18.6 | $ 21.2

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