Stock-based Compensation (Summary of Restricted Stock Activity) (Detail) (USD $)	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Restricted Stock Units (RSUs) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning of period, Shares	525,980	750,837
Granted, Shares	94,240	130,912
Vested, Shares	(202,006)	(259,440)
Forfeited, Shares	(9,300)	(37,257)
Outstanding, end of period, Shares	408,914	585,052
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	$ 41.20	$ 40.15
Granted, Weighted Average Grant Date Fair Value	$ 58.02	$ 42.80
Vested, Weighted Average Grant Date Fair Value	$ 42.09	$ 39.60
Forfeited, Weighted Average Grant Date Fair Value	$ 40.73	$ 40.49
Outstanding, end of period, Weighted Average Grant Date Fair Value	$ 44.65	$ 40.96
Performance Shares [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning of period, Shares	184,626	132,775
Granted, Shares	60,338	79,850
Vested, Shares	(22,826)
Forfeited, Shares	(3,800)	(22,700)
Outstanding, end of period, Shares	218,338	189,925
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	$ 40.42	$ 39.97
Granted, Weighted Average Grant Date Fair Value	$ 55.73	$ 41.67
Vested, Weighted Average Grant Date Fair Value	$ 44.78
Forfeited, Weighted Average Grant Date Fair Value	$ 37.90	$ 40.83
Outstanding, end of period, Weighted Average Grant Date Fair Value	$ 44.24	$ 40.58