Investments - Summary of Gross Real Estate and Loan Activity (Footnote) (Details) - USD ($) $ in Millions |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|
| Investments in and Advances to Affiliates, Schedule of Investments [Abstract] | |||
| Total accumulated depreciation and amortization associated with dispositions of real estate | $ 57.1 | $ 126.4 | |
| Revenue producing capitalized expenditures | 42.6 | 20.5 | |
| Non-revenue producing capitalized maintenance expenditures | 3.5 | 6.6 | |
| Discontinued Operations, Held-for-use | |||
| Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] | |||
| Gain on disposal of assets for properties | 24.6 | 40.5 | $ 35.8 |
| Discontinued Operations, Held-for-sale | |||
| Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] | |||
| Gain on disposal of assets for properties | $ 16.6 | $ 34.5 | $ 33.9 |