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Investments - Summary of Activity in Real Estate Assets Held for Sale (Footnote) (Details) (USD $)
In Millions, unless otherwise specified |
Dec. 31, 2014
Property
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Dec. 31, 2013
Property
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Dec. 31, 2012
Property
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|---|---|---|---|
| Schedule of Investments [Line Items] | |||
| Number of properties held for sale | 24 | 11 | 7 |
| Continuing Operations [Member] | |||
| Schedule of Investments [Line Items] | |||
| Number of properties held for sale | 19 | ||
| Net carrying amount of properties held for sale | 110.9 |