Share-Based Compensation (Details) (USD $) In Millions, except Share data, unless otherwise specified	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010
Options
Options exercised to purchase common shares ("options") (in shares)	185,714	271,242
Performance share units
Share-Based Compensation
Stock awards granted (in shares or units)	56,883
Potential earned PSUs payout for defined levels of performance under awards
Earned PSUs payout (as a percent of PSUs granted) on 75th or greater percentile rank	200.00%
Earned PSUs payout (as a percent of PSUs granted) on 50th percentile rank	100.00%
Earned PSUs payout (as a percent of PSUs granted) on 25th percentile rank	50.00%
Performance share units granted on percentile rank below 25th (as a percent)	0.00%
Assumptions used to value stock awards
Baseline value per common share (in dollars per share)	$ 35.17
Expected volatility for common shares (as a percent)	61.10%
Risk-free interest rate (as a percent)	1.32%
Other Share-Based Compensation Disclosures
Performance period of the award (in years)	three years
Grant date fair value (in dollars per share)	$ 49.15
The number of percentile ranks to fall between to earn interpolated PSUs, conditioned on the percentile rank exceeding 25%	2
Restricted shares
Share-Based Compensation
Stock awards granted (in shares or units)	314,359
Other Share-Based Compensation Disclosures
Grant date fair value (in dollars per share)	$ 33.92
Vesting period of awards to non employee Trustees	first anniversary of the grant date
Restricted common shares, forfeiture restrictions lapsed (in shares)	317,685
Weighted average grant date fair value of restricted common share, forfeiture restrictions lapsed (in dollars per share)	$ 32.94
Aggregate intrinsic value of restricted common shares, forfeiture restrictions lapsed	$ 11.1
Options
Options
Options exercised to purchase common shares ("options") (in shares)	185,714
Weighted average exercise price of options (in dollars per share)	$ 12.85
Aggregate intrinsic value of options exercised	$ 3.9

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The total intrinsic value of equity-based payment equity instruments, excluding stock (or unit) options, that vested during the reporting period as calculated by applying the disclosed pricing methodology.

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The period over which restricted shares granted to nonemployee Trustees will vest provided that the Trustee remains in his or her position.

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Description of the estimated period of time over which an employee is required to provide service in exchange for the equity-based payment award, which often is the vesting period. This period may be explicit or implicit based on the terms of the award, and may be presented in a variety of ways (for example, year, month and year, day, month and year, quarter of a year).

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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).

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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).

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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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.

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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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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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The total accumulated difference between fair values of underlying shares on dates of exercise and exercise price on options which were exercised (or share units converted) into shares during the reporting period under the plan.

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The weighted average price at which option holders acquired shares when converting their stock options into shares under the plan during the reporting period.

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