Debt - Credit Facility - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Oct. 31, 2014	Mar. 21, 2012
Line of Credit Facility [Line Items]
Letter of credit payable			$ 79,708,000
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	$ 89,300,000
For the period from September 15, 2014 through and including March 21, 2015 [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Ticking fee rate	0.00%
For the period from March 22, 2015 through and including March 21, 2016 [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Ticking fee rate	1.00%
For the period from March 22, 2016 through and including March 21, 2017 [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Ticking fee rate	2.00%
For the period from March 22, 2017 through and including March 21, 2018 [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Ticking fee rate	3.00%
For the period from March 22, 2018 through and including March 21, 2019 [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Ticking fee rate	4.00%
For the period from March 22, 2019 through November 15, 2019 [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Ticking fee rate	5.00%
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
Amended Credit Agreement [Member]
Line of Credit Facility [Line Items]
Effective interest rate on borrowings		4.90%
Amended Credit Agreement [Member] | Previously Issued by J P Morgan Chase Bank [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Aggregate letters of credit previously issued	$ 2,352,000
Financing Agreement [Member]
Line of Credit Facility [Line Items]
Effective interest rate on borrowings	9.50%