Senior Notes - Schedule of Carrying Amount of Senior Notes (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 5.375% Senior Notes Due 2014 [Member]	Dec. 31, 2012 5.375% Senior Notes Due 2014 [Member]	Mar. 31, 2013 5.375% Senior Notes Due 2015 [Member]	Dec. 31, 2012 5.375% Senior Notes Due 2015 [Member]	Mar. 31, 2013 5.625% Senior Notes Due 2020 [Member]	Dec. 31, 2012 5.625% Senior Notes Due 2020 [Member]	Mar. 31, 2013 6.000% Senior Notes due 2043 [Member]	Jan. 10, 2013 6.000% Senior Notes due 2043 [Member]
Schedule Of Carrying Amount Of Senior Notes [Line Items]
Total Debt Obligations	$ 995,032	$ 744,842	$ 249,668	$ 249,621	$ 249,905	$ 249,895	$ 245,459	$ 245,326	$ 250,000	$ 250,000