Share-Based Compensation Stock option transactions, exercise price - (Detail) (USD $)	3 Months Ended
	Mar. 31, 2012 Years	Mar. 31, 2012 Options Outstanding Exercise Price Range 1 [Member] Years	Mar. 31, 2012 Options Outstanding Exercise Price Range 2 [Member] Years	Mar. 31, 2012 Options Outstanding Exercise Price Range 3 [Member] Years	Mar. 31, 2012 Minimum [Member]	Dec. 31, 2011 Minimum [Member]	Mar. 31, 2012 Minimum [Member] Options Outstanding Exercise Price Range 1 [Member]	Mar. 31, 2012 Minimum [Member] Options Outstanding Exercise Price Range 2 [Member]	Mar. 31, 2012 Minimum [Member] Options Outstanding Exercise Price Range 3 [Member]	Mar. 31, 2012 Maximum [Member]	Dec. 31, 2011 Maximum [Member]	Mar. 31, 2012 Maximum [Member] Options Outstanding Exercise Price Range 1 [Member]	Mar. 31, 2012 Maximum [Member] Options Outstanding Exercise Price Range 2 [Member]	Mar. 31, 2012 Maximum [Member] Options Outstanding Exercise Price Range 3 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Unvested stock options decrease in period (in shares)	88,396
Stock-option transactions under the terms of the LTSIP [Roll Forward]
Options outstanding, beginning balance (in shares)	1,773,635	756,811	539,174	328,533
Exercised (in shares)	(149,117)
Options outstanding, ending balance (in shares)	1,624,518	756,811	539,174	328,533
ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosures [Roll Forward]
Weighted-average exercise price, options outstanding, beginning of period (in dollars per share)	$ 8.54	$ 4.82	$ 11.78	$ 13.52	$ 4.37	$ 3.70	$ 4.37	$ 11.40	$ 13.10	$ 17.35	$ 17.35	$ 7.84	$ 12.43	$ 17.35
Weighted-average exercise price of options exercised during the period (in dollars per share)	$ 4.71				$ 3.70					$ 13.10
Weighted-average exercise price, options outstanding, end of period (in dollars per share)	$ 8.89	$ 4.82	$ 11.78	$ 13.52	$ 4.37	$ 3.70	$ 4.37	$ 11.40	$ 13.10	$ 17.35	$ 17.35	$ 7.84	$ 12.43	$ 17.35
Weighted-average remaining term (in years)	2.3	1.1	2.7	4.3
Number of exercisable options at end of period	1,595,120	756,811	539,174	299,135
Weighted-average exercise price, options exercisable	$ 8.81	$ 4.82	$ 11.78	$ 13.56
Number of unvested options at end of period	29,398	0	0	29,398
Weighted-average exercise price, unvested options	$ 13.10	$ 0.00	$ 0.00	$ 13.10