Stock-based Compensation Plans - Non-Vested Restricted Stock Outstanding (Detail) (USD $)	9 Months Ended
	Sep. 30, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted	$ 21.32
Vested, Shares	(55,588)
Equity Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance, Weighted Average Grant Date Fair Value	$ 49.64
Granted	$ 63.37
Vested	$ 56.83
Forfeited	$ 58.37
Ending Balance, Weighted Average Grant Date Fair Value	$ 58.56
Beginning Balance, Shares	221,609
Granted, Shares	359,732
Vested, Shares	(140,818)
Forfeited, Shares	(23,782)
Ending Balance, Shares	416,741
Liability Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance, Weighted Average Grant Date Fair Value	$ 48.76
Granted	$ 63.90
Vested	$ 51.30
Forfeited	$ 50.50
Ending Balance, Weighted Average Grant Date Fair Value	$ 57.76
Beginning Balance, Shares	487,244
Granted, Shares	370,385
Vested, Shares	(324,433)
Forfeited, Shares	(2,093)
Ending Balance, Shares	531,103