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
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

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.

No definition available.

Name:	us-gaap_GainLossOnSaleOfBusiness
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration