Stock Plans - Share Based Compensation Activity (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding Balance, Shares	872,049
Stock options granted	0
Exercised, Shares	(572,337)
Canceled, Shares	(17,410)
Outstanding Balance, Shares	282,302
Exercisable Balance, Shares	153,289
Outstanding Balance, Weighted Average Exercise Price	$ 25.99
Granted, Weighted Average Exercise Price	$ 0
Exercised, Weighted Average Exercise Price	$ 23.56
Canceled, Weighted Average Exercise Price	$ 45.25
Outstanding Balance, Weighted Average Exercise Price	$ 30.88
Exercisable Balance, Weighted Average Exercise Price	$ 35.58
Equity Incentive Plan Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested shares balance, Shares	610,227
Granted, Shares	155,478
Vested, Shares	(166,435)
Canceled, Shares	(9,407)
Nonvested shares balance, Shares	589,863	610,227
Nonvested shares balance, Weighted Average Grant-Date Fair-Value	$ 34.20
Granted, Weighted Average Grant-Date Fair-Value	$ 41.61	$ 43.56	$ 33.57
Vested, Weighted Average Grant-Date Fair-Value	$ 31.54
Canceled, Weighted Average Grant-Date Fair-Value	$ 41.47
Nonvested shares balance, Weighted Average Grant-Date Fair-Value	$ 36.61	$ 34.20