Indebtedness (Unsecured lines of credit) - Additional Information (Detail) $ in Thousands	1 Months Ended	3 Months Ended	9 Months Ended
	Jan. 31, 2015 USD ($) OptionPlan	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)
Debt Instrument [Line Items]
Letters of credit issued			$ 122
Incurred fees and expenses			4,008
Net gain (loss) on extinguishment of indebtedness		$ (14,070)	(197)	$ (18,357)
Cash Management Line [Member]
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity	$ 30,000		30,000
Number of extension options | OptionPlan	1
Time period extension option	1 year
Lines of Credit [Member]
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity			$ 300,000
Interest Rate, spread over LIBOR			1.05%
Line of credit facility annual facility fees percentage			0.20%
Interest rate based on credit ratings ranges, minimum			0.875%
Interest rate based on credit ratings ranges, maximum			1.55%
Facility fee rate based on credit ratings range, minimum			0.125%
Facility fee rate based on credit ratings range, maximum			0.30%
Line of credit facility, competitive bid option for short-term funds, percentage			50.00%