Debt - Credit Facility - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 31, 2014	Mar. 21, 2012	Jun. 30, 2014
Line of Credit Facility [Line Items]
Letter of credit payable				$ 79,708,000
Effective interest rate on borrowings	4.90%	3.60%	2.40%
Line of Credit [Member]
Line of Credit Facility [Line Items]
Letter of credit payable				79,708,000
Number of mortgages parcels of land owned	30
Percentage of cash collateral amount equal to face amount of ED Letter of Credit	109.00%
Percentage of cash collateral amount equal to face amount	103.00%
Cash collateral for outstanding letters of credit	89,304,000
Line of Credit [Member] | Scenario, Forecast [Member] | For the period from September 15, 2014 through and including March 21, 2015 [Member]
Line of Credit Facility [Line Items]
Ticking fee rate	0.00%
Line of Credit [Member] | Scenario, Forecast [Member] | For the period from March 22, 2015 through and including March 21, 2016 [Member]
Line of Credit Facility [Line Items]
Ticking fee rate	1.00%
Line of Credit [Member] | Scenario, Forecast [Member] | For the period from March 22, 2016 through and including March 21, 2017 [Member]
Line of Credit Facility [Line Items]
Ticking fee rate	2.00%
Line of Credit [Member] | Scenario, Forecast [Member] | For the period from March 22, 2017 through and including March 21, 2018 [Member]
Line of Credit Facility [Line Items]
Ticking fee rate	3.00%
Line of Credit [Member] | Scenario, Forecast [Member] | For the period from March 22, 2018 through and including March 21, 2019 [Member]
Line of Credit Facility [Line Items]
Ticking fee rate	4.00%
Line of Credit [Member] | Scenario, Forecast [Member] | For the period from March 22, 2019 through November 15, 2019 [Member]
Line of Credit Facility [Line Items]
Ticking fee rate	5.00%
Line of Credit [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Percentage of cash collateral amount equal to face amount of ED Letter of Credit	109.00%
Percentage of cash collateral amount equal to face amount	103.00%
Credit Agreement [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity					325,000,000
Revolving credit facility, maturity date	Mar. 21, 2015
Credit Agreement [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Letter of credit agreement borrowing capacity						25,000,000
Amended Credit Agreement [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Revised maximum borrowing capacity						135,000,000
Letter of credit agreement borrowing capacity						85,000,000
Amended Credit Agreement [Member] | Line of Credit [Member] | Previously Issued by J P Morgan Chase Bank [Member]
Line of Credit Facility [Line Items]
Letter of credit agreement borrowing capacity	2,352,000