Long-Term Debt and Notes Payable - Additional Information (Detail) (USD $) In Millions, unless otherwise specified		1 Months Ended	6 Months Ended						6 Months Ended			12 Months Ended
	Apr. 30, 2014	Apr. 30, 2014 Senior Notes [Member]	Jun. 30, 2014 Senior Notes [Member]	Dec. 31, 2013 Unsecured bilateral letters of credit [Member]	Jun. 30, 2014 Letter of Credit Facility [Member]	Apr. 30, 2014 Letter of Credit Facility [Member]	Apr. 30, 2014 Term Loan [Member]	Apr. 30, 2014 Tangible Equity Units [Member]	Jun. 30, 2014 Revolving Credit Facility [Member]	Mar. 31, 2014 Revolving Credit Facility [Member]	Dec. 31, 2013 Revolving Credit Facility [Member]	Dec. 31, 2013 Former Credit Agreement [Member]
Debt Instrument [Line Items]
Letter of credit, first lien	$ 950.0					$ 400.0	$ 300.0
Issue of second-lien seven-year senior secured notes		500.0	500.0
Issue of tangible equity units								287.5
Tangible equity units, aggregate principal amount		47.5	47.5
Unamortized issuance fees recognized as expenses									28.0
Line of credit aggregate borrowing capacity					100.0				950.0
Letter of credit outstanding borrowings				96.9					214.3	250.0	735.7	80.0
Average outstanding borrowings under the Former Credit Agreement												$ 23.5
Average interest rate												2.28%