Summary of Restricted Stock Activity (Detail) (USD $)	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
Time-Based Restricted Stock Units
Schedule of Share based Compensation Arrangements by Share based Payment Award, Equity Instruments, Other Than Options, Restricted Stock Units [Line Items]
Outstanding, beginning of period, Shares	768,529	838,129
Granted, Shares	172,691	111,496
Vested, Shares	(121,522)	(187,123)
Forfeited, Shares	(26,705)	(21,944)
Outstanding, end of period, Shares	792,993	740,558
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	$ 40.17	$ 40.93
Granted, Weighted Average Grant Date Fair Value	$ 36.99	$ 46.10
Vested, Weighted Average Grant Date Fair Value	$ 35.30	$ 45.14
Forfeited, Weighted Average Grant Date Fair Value	$ 40.03	$ 41.54
Outstanding, end of period, Weighted Average Grant Date Fair Value	$ 40.22	$ 40.63
Performance And Market-Based Restricted Stock Units
Schedule of Share based Compensation Arrangements by Share based Payment Award, Equity Instruments, Other Than Options, Restricted Stock Units [Line Items]
Outstanding, beginning of period, Shares	69,500	101,680
Granted, Shares	94,000	42,500
Vested, Shares		(25,055)
Forfeited, Shares	(1,125)
Outstanding, end of period, Shares	162,375	119,125
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	$ 45.37	$ 39.62
Granted, Weighted Average Grant Date Fair Value	$ 36.75	$ 46.47
Vested, Weighted Average Grant Date Fair Value		$ 45.21
Forfeited, Weighted Average Grant Date Fair Value	$ 41.16
Outstanding, end of period, Weighted Average Grant Date Fair Value	$ 40.34	$ 40.89