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Net Income Per Share - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Stock options and stock settled appreciation rights (SSAR's) excluded from calculation of diluted shares		132,200		0
Purchased Restricted Stock outstanding as percentage of common stock			0.70%	0.60%
Net income allocated to PRS	$ 0.6	$ 0.1	$ 1.9	$ 1.2
Maximum [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Difference amount between basic and diluted net income per share	$ 0.01	$ 0.01	$ 0.01	$ 0.01

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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