Stock Plans - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended			9 Months Ended		12 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2012 REIT Conversion [Member]	Sep. 30, 2012 REIT Conversion [Member]	Sep. 30, 2013 Equity Incentive Plan Restricted Stock [Member]	Dec. 31, 2012 Equity Incentive Plan Restricted Stock [Member]	Sep. 30, 2013 Restricted Stock Awards for Certain Management [Member]	Sep. 30, 2011 Restricted Stock Awards for Certain Management [Member]	Dec. 31, 2011 Restricted Stock Awards for Certain Management [Member]	Sep. 30, 2013 Restricted Stock Awards for Certain Management [Member] Minimum [Member]	Sep. 30, 2013 Restricted Stock Awards for Certain Management [Member] Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock award granted by Company									37,000	67,400
Vesting period from the date of grant									May vest in 2016	May vest in 2014
Performance period									3 years
Weighted-average grant-date fair value of restricted stock awards granted									$ 45.01		$ 44.39
Risk-free rate of weighted-average grant date fair value									0.40%
Volatility rate of weighted-average grant date fair value												23.00%	64.00%
Expected life assumption for weighted average grant date fair value									3 years
Stock option Cancelled, result of the REIT conversion					167,500	167,500
Restricted Stock Units Cancelled, result of the REIT conversion					401,000	401,000
Compensation Costs reversed					$ 2.1	$ 2.1
Restricted stock award, outstanding							527,153	574,933
Compensation cost on stock-based compensation plans	$ 1.8	$ (0.2)	$ 6.4	$ 5.1