Carrying Values and Fair Values of Debt Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Senior Notes due 2015		Dec. 31, 2011 Senior Notes due 2015		Jul. 15, 2008 Senior Notes due 2015	Dec. 31, 2012 Senior Notes due 2016		Dec. 31, 2011 Senior Notes due 2016		Apr. 02, 2011 Senior Notes due 2016	Dec. 31, 2012 Senior Notes due 2018		Dec. 31, 2011 Senior Notes due 2018		Apr. 02, 2011 Senior Notes due 2018	Dec. 31, 2012 Senior Notes due 2019		Dec. 31, 2011 Senior Notes due 2019		Oct. 26, 2009 Senior Notes due 2019	Dec. 31, 2012 Senior Notes due 2021		Dec. 31, 2011 Senior Notes due 2021		Apr. 02, 2011 Senior Notes due 2021
Debt Instrument [Line Items]
Carrying value of senior notes	$ 2,238,935	$ 1,178,657	$ 175,000		$ 175,000		$ 175,000	$ 100,000		$ 100,000		$ 100,000	$ 50,000		$ 50,000		$ 50,000	$ 175,000		$ 175,000		$ 175,000	$ 100,000		$ 100,000		$ 100,000
Fair value of senior notes			$ 193,949	[1]	$ 186,305	[1]		$ 103,068	[1]	$ 98,980	[1]		$ 52,476	[1]	$ 51,220	[1]		$ 195,584	[1]	$ 174,125	[1]		$ 107,418	[1]	$ 104,250	[1]

[1] Senior Notes are classified as Level 2 within the fair value hierarchy. Fair value is based on quotes of bonds with similar ratings in similar industries.