Fair Value Measurements - Estimated Fair Value of Senior Notes (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule Of Fair Value Of Senior Notes [Line Items]
Carrying Amount	$ 1,095,225	$ 744,842
Fair Value	1,108,513	809,200
5.375% Senior Notes due 2014 [Member]
Schedule Of Fair Value Of Senior Notes [Line Items]
Carrying Amount	249,716	249,621
Fair Value	260,213	267,208
5.375% Senior Notes due 2015 [Member]
Schedule Of Fair Value Of Senior Notes [Line Items]
Carrying Amount	249,915	249,895
Fair Value	263,550	268,867
5.625% Senior Notes due 2020 [Member]
Schedule Of Fair Value Of Senior Notes [Line Items]
Carrying Amount	245,594	245,326
Fair Value	266,250	273,125
6.000% Senior Notes due 2043 [Member]
Schedule Of Fair Value Of Senior Notes [Line Items]
Carrying Amount	350,000
Fair Value	$ 318,500