Long-Term Debt (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
Debt Instrument [Line Items]
Long-term debt	$ 2,100,000	$ 1,600,000	$ 1,650,000
Less: current portion			(50,000)
Total long-term debt	2,100,000	1,600,000	1,600,000
Medium-term notes
Debt Instrument [Line Items]
Long-term debt			50,000
2010 Senior Notes
Debt Instrument [Line Items]
Long-term debt	500,000	500,000	500,000
2011 Senior Notes
Debt Instrument [Line Items]
Long-term debt	600,000	600,000	600,000
2013 Senior Notes
Debt Instrument [Line Items]
Long-term debt	500,000	500,000	500,000
2014 Senior Notes
Debt Instrument [Line Items]
Long-term debt	$ 500,000