Computation of Basic and Diluted Earnings per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Net Income (Loss) Available to Common Stockholders, Basic
Net income	$ 20,980us-gaap_NetIncomeLoss	$ 21,393us-gaap_NetIncomeLoss
Weighted Average Number of Shares Outstanding, Basic and Diluted
Weighted average common shares outstanding for basic earnings per share computation	45,378,230	43,997,815
Dilutive effect of stock-based compensation	461,961	622,961
Weighted average common shares outstanding for diluted earnings per common share computation	45,840,191	44,620,776
Earnings Per Share, Basic and Diluted
Basic earnings per common share	$ 0.46us-gaap_EarningsPerShareBasic	$ 0.49us-gaap_EarningsPerShareBasic
Diluted earnings per common share	$ 0.46us-gaap_EarningsPerShareDiluted	$ 0.48us-gaap_EarningsPerShareDiluted
Stock Options
Earnings Per Share, Basic and Diluted [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	5,000	0
Unvested Restricted Stock
Earnings Per Share, Basic and Diluted [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	166,770	111,417

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Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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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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