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Earnings Per Share - Summary of Earnings Per Share (Detail) (USD $)
In Millions, except Share data in Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Numerator (in millions):    
Net income available to common stockholders $ 17.9us-gaap_NetIncomeLoss $ 16.9us-gaap_NetIncomeLoss
Denominator (in thousands):    
Weighted average common shares outstanding 24,301us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 24,362us-gaap_WeightedAverageNumberOfSharesOutstandingBasic
Dilutive effect of stock options and awards 507us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements 273us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements
Denominator for diluted earnings per share 24,808us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 24,635us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding
Net income per share, basic: $ 0.74us-gaap_EarningsPerShareBasic $ 0.69us-gaap_EarningsPerShareBasic
Net income per share, diluted: $ 0.72us-gaap_EarningsPerShareDiluted $ 0.69us-gaap_EarningsPerShareDiluted