Long-Term Debt (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Debt Instrument [Line Items]
Long-term debt	$ 1,650,000	$ 1,500,000	$ 1,550,000
Less: current portion	(50,000)	(400,000)	(400,000)
Total long-term debt	1,600,000	1,100,000	1,150,000
Medium-Term Notes
Debt Instrument [Line Items]
Long-term debt	50,000	50,000	100,000
2008 Senior Notes
Debt Instrument [Line Items]
Long-term debt		350,000	350,000
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