Summary of Non Vested Restricted Stock Unit Activity (Detail) (USD $) In Thousands, except Share data, unless otherwise specified				12 Months Ended
	Dec. 31, 2012 Y	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Restricted Stock Units (RSUs) Y
Schedule Of Activity Related To Restricted Shares And Restricted Share [Line Items]
Number of Non-Vested Shares, Outstanding, beginning of the year				334,459
Number of Non-Vested Shares, Granted				178,690
Number of Non-Vested Shares, Vested				(78,753)
Number of Non-Vested Shares, Forfeited				(69,608)
Number of Non-Vested Shares, Outstanding, end of the year				364,788
Weighted Average Grant Date Price, Outstanding, beginning of the year	$ 55.77	$ 56.60	$ 57.09	$ 56.60
Weighted Average Grant Date Price, Granted				$ 56.95
Weighted Average Grant Date Price, Vested				$ 45.54
Weighted Average Grant Date Price, Forfeited				$ 54.92
Weighted Average Grant Date Price, Outstanding, end of the year	$ 55.77	$ 56.60	$ 57.09	$ 55.77
Weighted Average Remaining Contractual Life In Years, Outstanding, end of the year	7			1.3
Aggregate Intrinsic Value, Outstanding, end of the year				$ 26,013