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Summary of Significant Accounting Policies (Narrative) (Details) - New Accounting Pronouncement, Early Adoption, Effect - Accounting Standards Update 2017-05 - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
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Sep. 30, 2017 | Sep. 30, 2017
New Accounting Pronouncement, Early Adoption [Line Items] |  |
Gain on sale of ground lease business | $ 178.9 | $ 178.9
Expected Incremental Gain to be Recorded Retrospectively |  |
New Accounting Pronouncement, Early Adoption [Line Items] |  |
Gain on sale of ground lease business | $ 55.5 | $ 55.5

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- Definition Gain (loss) from sale or disposal of an organization or integrated set of activities (for example, but not limited to, a partnership or corporation) engaged in providing a product or service in a commercial, industrial, or professional environment. + References No definition available. + Details
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