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Share and Unit-Based Plans: Long-Term Incentive Plan Units (Details)											12 Months Ended
	Jan. 07, 2016	Dec. 31, 2015 shares	Jan. 12, 2015	Jan. 01, 2015 $ / shares shares	Dec. 31, 2014 shares	Mar. 07, 2014 $ / shares shares	Jan. 16, 2014	Jan. 01, 2014 $ / shares shares	Dec. 31, 2013 shares	Feb. 15, 2013 $ / shares shares	Dec. 31, 2015 $ / shares shares	Dec. 31, 2014 $ / shares shares	Dec. 31, 2013 $ / shares shares
Stock units
Share and unit-based plans
Conversion rate											1
Granted (in shares)											77,282	75,309	67,920
Granted (in dollars per share) | $ / shares											$ 86.53	$ 60.50	$ 62.01
Vested (in shares)											86,761	68,253	45,279
Forfeited (in shares)											2,809	0	0
LTIP units
Share and unit-based plans
Conversion rate											1
Granted (in shares)											424,442	725,908	332,189
Granted (in dollars per share) | $ / shares											$ 74.71	$ 51.71	$ 66.58
Absolute return requirement (in shares)				3.00%
Level of percentile ranking at which awards vested (as a percent)			150.00%				96.00%
Vested (in shares)									318,900		414,822	679,213	518,900
Forfeited (in shares)									13,289		0	0	13,289
Risk free rate (as a percent)				0.25%
Volatility rate (as a percent)				16.81%
Long Term Incentive Plan, Market Indexed
Share and unit-based plans
Vested (in shares)		55,934			136,465
Executive Officer | LTIP units
Share and unit-based plans
Granted (in shares)										332,189
Granted (in dollars per share) | $ / shares						$ 60.25				$ 66.58
Executive Officer | LTIP units | 2014 LTIP Units Series 2
Share and unit-based plans
Granted (in shares)								70,042
Granted (in dollars per share) | $ / shares								$ 58.89
Executive Officer | LTIP units | 2014 LTIP Units Series 1
Share and unit-based plans
Granted (in shares)						246,471		272,930
Granted (in dollars per share) | $ / shares								$ 45.34
Executive Officer | LTIP units | 2015 LTIP Units Series 1 [Member]
Share and unit-based plans
Granted (in shares)				49,451
Granted (in dollars per share) | $ / shares				$ 83.41
Executive Officer | LTIP units | 2015 LTIP Units Series 2
Share and unit-based plans
Granted (in shares)				186,450
Granted (in dollars per share) | $ / shares				$ 66.37
Subsequent Event | LTIP units
Share and unit-based plans
Level of percentile ranking at which awards vested (as a percent)	130.00%

X

- Definition

Represents the level of percentile ranking terms of total return per common stock share to the total return of a group of peer REITs at which the equity instruments other than options vested during the period.

+ References

No definition available.

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments, Other than Options, Conversion Rate

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Share-based Compensation Arrangement by Share-based Payment Award Equity Instruments other than Options, Absolute Return Requirement

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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting 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 (c)(1)(iv)(3)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ 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 (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ 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 (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting 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 (c)(2)(iii)(2)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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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

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-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=6415400&loc=d3e5070-113901

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(iv)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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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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