Share-Based Compensation - Performance and Restricted Unit Activity (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding beginning balance, Options	553,049
Granted, Shares	15,000
Outstanding ending balance, Options	453,070
Outstanding beginning balance, Weighted average exercise price	$ 20.91
Weighted average grant date fair value, Granted	$ 34.82
Outstanding ending balance, Weighted average exercise price	$ 21.78
Aggregate intrinsic value, Outstanding ending balance	$ 4,571
Performance Based Restricted Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding beginning balance, Options
Granted, Shares	213,022
Vested, Shares	(31,285)
Outstanding ending balance, Options	181,737
Performance and restricted units expected to vest ending balance, Shares	181,737
Outstanding beginning balance, Weighted average exercise price
Weighted average grant date fair value, Granted	$ 33.59
Weighted average grant date fair value, Vested	$ 33.73
Outstanding ending balance, Weighted average exercise price	$ 33.56
Weighted average grant date fair value, Performance and restricted units expected to vest ending balance	$ 33.56
Aggregate intrinsic value, Vested	823
Aggregate intrinsic value, Outstanding ending balance	4,571
Aggregate intrinsic value, Performance and restricted units expected to vest ending balance	$ 4,571
Weighted average remaining contractual term, Performance and restricted units expected to vest ending balance	3 months 18 days