Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Earnings Per Share [Abstract]
Net income attributable to common shareholders	$ 16,122	$ 15,766
Weighted average common shares outstanding - basic	110,916	110,257
Net effect of dilutive securities:
Contingently issuable performance shares and restricted stock units	461	631
Weighted average common shares outstanding — diluted	111,377	110,888
Earnings per average common share attributable to common shareholders -basic (in dollars per share)	$ 0.15us-gaap_EarningsPerShareBasic	$ 0.14us-gaap_EarningsPerShareBasic
Earnings per average common share attributable to common shareholders -diluted (in dollars per share)	$ 0.14us-gaap_EarningsPerShareDiluted	$ 0.14us-gaap_EarningsPerShareDiluted

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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Number of shares of restricted stock determined by relating the portion of time within a reporting period that restricted shares have been outstanding to the total time in that period. Restricted shares are subject to sales, contractual, regulatory or other restrictions that prevent or inhibit the holder from freely disposing of them before the restriction ends.

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