Unvested Share Activity (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Shares
Granted, Shares	200,000	200,000	300,000
Weighted-Average Grant Date Fair Value
Nonvested, Weighted-Average Grant Date Fair Value, Beginning Balance	$ 40.27
Granted, Weighted-Average Grant Date Fair Value	$ 56.20
Vested, Weighted-Average Grant Date Fair Value	$ 38.50
Forfeited, Weighted-Average Grant Date Fair Value	$ 53.58
Nonvested, Weighted-Average Grant Date Fair Value, Ending Balance	$ 40.27	$ 40.27
Unvested Shares
Shares
Nonvested, Beginning Balance	589,986
Granted, Shares	232,418
Vested, Shares	(154,795)
Forfeited, Shares	(48,930)
Nonvested, Ending Balance	618,679