Net Income Per Common Share - Net Income Per Common Share on Basic and Diluted Basis (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Earnings Per Share [Abstract]
Net income	$ 58.8us-gaap_NetIncomeLoss	$ 40.2us-gaap_NetIncomeLoss
Weighted average common shares outstanding - basic	28,129,692	29,165,911
Effect of dilutive securities:
Equity compensation awards	547,687	497,179
Weighted average common shares outstanding - diluted	28,677,379	29,663,090
Net income per common share - basic	$ 2.09us-gaap_EarningsPerShareBasic	$ 1.38us-gaap_EarningsPerShareBasic
Net income per common share - diluted	$ 2.05us-gaap_EarningsPerShareDiluted	$ 1.36us-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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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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