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Summary of Assumptions Used in Computing fair value of the PSUs (Parenthetical) (Detail) | 12 Months Ended
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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 + Details
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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. + References No definition available. + Details
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