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Earnings Per Share - Summary of Earnings Per Share (Detail) (USD $)
In Millions, except Share data in Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Numerator (in millions):      
Net income available to common stockholders $ 84.1us-gaap_NetIncomeLoss $ 77.8us-gaap_NetIncomeLoss $ 66.9us-gaap_NetIncomeLoss
Denominator (in thousands):      
Weighted average common shares outstanding 24,391us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 23,651us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 23,187us-gaap_WeightedAverageNumberOfSharesOutstandingBasic
Dilutive effect of stock options and awards 487us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements 505us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements 663us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements
Denominator for diluted earnings per share 24,878us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 24,156us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 23,850us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding
Net income per share, basic: $ 3.45us-gaap_EarningsPerShareBasic $ 3.29us-gaap_EarningsPerShareBasic $ 2.89us-gaap_EarningsPerShareBasic
Net income per share, diluted: $ 3.38us-gaap_EarningsPerShareDiluted $ 3.22us-gaap_EarningsPerShareDiluted $ 2.81us-gaap_EarningsPerShareDiluted