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Summary of Assumptions Used in Computing fair value of the PSUs (Parenthetical) (Detail)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Volatility			34.00%
Performance Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Volatility	[1]	27.00%
Minimum | Performance Share Units | Peer Group and NAREIT
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Volatility		19.00%
Maximum | Performance Share Units | Peer Group and NAREIT
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Volatility		28.00%
[1] This represents the volatility assumption used for IRT. The volatility assumptions used for our peer group and the NAREIT Mortgage Index ranged from 19% to 28%

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- Definition

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(ii)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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