Long-term Debt Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Long Term Debt Maturities Repayments Of Principal [Line Items]
Amount of long-term debt maturing - 2013	$ 400,000
Amount of long-term debt maturing - 2014
Amount of long-term debt maturing - 2015
Amount of long-term debt maturing - 2016	300,000
Amount of long-term debt maturing - 2017
Amount of long-term debt maturing - Thereafter	800,000
Long-term debt	1,500,000	1,550,000
Medium-Term Notes
Long Term Debt Maturities Repayments Of Principal [Line Items]
Amount of long-term debt maturing - 2013	50,000
Amount of long-term debt maturing - 2014
Amount of long-term debt maturing - 2015
Amount of long-term debt maturing - 2016
Amount of long-term debt maturing - 2017
Amount of long-term debt maturing - Thereafter
Long-term debt	50,000	100,000
2008 Senior Notes
Long Term Debt Maturities Repayments Of Principal [Line Items]
Amount of long-term debt maturing - 2013	350,000
Amount of long-term debt maturing - 2014
Amount of long-term debt maturing - 2015
Amount of long-term debt maturing - 2016
Amount of long-term debt maturing - 2017
Amount of long-term debt maturing - Thereafter
Long-term debt	350,000	350,000
2010 Senior Notes
Long Term Debt Maturities Repayments Of Principal [Line Items]
Amount of long-term debt maturing - 2013
Amount of long-term debt maturing - 2014
Amount of long-term debt maturing - 2015
Amount of long-term debt maturing - 2016
Amount of long-term debt maturing - 2017
Amount of long-term debt maturing - Thereafter	500,000
Long-term debt	500,000	500,000
2011 Senior Notes
Long Term Debt Maturities Repayments Of Principal [Line Items]
Amount of long-term debt maturing - 2013
Amount of long-term debt maturing - 2014
Amount of long-term debt maturing - 2015
Amount of long-term debt maturing - 2016	300,000
Amount of long-term debt maturing - 2017
Amount of long-term debt maturing - Thereafter	300,000
Long-term debt	$ 600,000	$ 600,000