Long-Term Debt and Notes Payable - Additional Information 1 (Detail) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Line of Credit Facility maximum amount outstanding	$ 207,800,000	$ 195,800,000
Letter of Credit Facility [Member]
Debt Instrument [Line Items]
Letter of credit, first lien	400,000,000	400,000,000
Letter of credit supported by Cash collateral	116,800,000	88,800,000
Financial Letter of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility maximum amount outstanding	0	0
Letter of credit supported by Cash collateral	$ 47,700,000	$ 19,700,000