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Summary of Significant Accounting Policies (New Accounting Pronouncements) (Details) - Accounting Standards Update 2017-05 - New Accounting Pronouncement, Early Adoption, Effect $ in Millions | 9 Months Ended
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Sep. 30, 2017 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |
Gain on sale of ground lease business | $ 178.9
Expected incremental gain to be recorded retrospectively |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |
Gain on sale of ground lease business | $ 55.5

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