Calculation of Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Calculation of Basic Earnings Per Share:
Net Income	$ 21,109us-gaap_NetIncomeLoss	$ 17,681us-gaap_NetIncomeLoss	$ 36,092us-gaap_NetIncomeLoss
Weighted shares outstanding - Basic (in shares)	14,604,841	14,487,646
Basic Earnings Per Share (in usd per share)	$ 1.45us-gaap_EarningsPerShareBasic	$ 1.22us-gaap_EarningsPerShareBasic
Reconciliation of Numerator:
Net Income	$ 21,109us-gaap_NetIncomeLoss	$ 17,681us-gaap_NetIncomeLoss	$ 36,092us-gaap_NetIncomeLoss
Reconciliation of Denominator:
Weighted shares outstanding - Basic (in shares)	14,604,841	14,487,646
Effect of dilutive securities:
Share-based Compensation (in shares)	51,469	52,505
Adjusted denominator-Diluted (in shares)	14,656,310	14,540,151
Diluted Earnings Per Share (in usd per share)	$ 1.44us-gaap_EarningsPerShareDiluted	$ 1.22us-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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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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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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