Indebtedness (Unsecured lines of credit) - Additional Information (Detail) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Jan. 31, 2015 USD ($) OptionPlans	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)
Debt Instrument [Line Items]
Letters of credit issued		$ 122		$ 122
Incurred fees and expenses				4,008
Net gain (loss) on extinguishment of indebtedness		0	$ (4,287)	(197)	$ (4,287)
Cash Management Line [Member]
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity	$ 30,000	30,000		30,000
Number of extension options | OptionPlans	1
Time period extension option	1 year
Lines of Credit [Member]
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity		$ 300,000		$ 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%