Debt - Credit Facility - Additional Information (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($) LettersOfCredit	Jun. 30, 2014	Jun. 30, 2015 USD ($) LettersOfCredit	Jun. 30, 2014	Oct. 31, 2014 USD ($)	Mar. 21, 2012 USD ($)
Line of Credit Facility [Line Items]
Aggregate letters of credit previously issued	$ 82,001,000		$ 82,001,000
Letter of credit payable					$ 79,708,000
Percentage of cash collateral amount equal to face amount of ED Letter of Credit	109.00%		109.00%
Percentage of cash collateral amount equal to face amount	103.00%		103.00%
Cash collateral for outstanding letters of credit	$ 89,300,000		$ 89,300,000
Reduction in cash collateral for outstanding letters of credit	$ 60,000		$ 60,000
Number of letters of credit terminated | LettersOfCredit	1		1
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Letters of credit terminated, amount	$ 59,000		$ 59,000
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%		109.00%
Percentage of cash collateral amount equal to face amount	103.00%		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%		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	$ 2,293,000		$ 2,293,000
Financing Agreement [Member]
Line of Credit Facility [Line Items]
Effective interest rate on borrowings	9.91%		9.91%