Carrying Values and Fair Values of Debt Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Debt Instrument [Line Items]
Carrying value of senior notes	$ 2,075,703		$ 2,238,935
6.22% Senior Notes due 2015
Debt Instrument [Line Items]
Carrying value of senior notes	175,000		175,000
Fair value of senior notes	190,158	[1]	193,949	[1]
3.30% Senior Notes due 2016
Debt Instrument [Line Items]
Carrying value of senior notes	100,000		100,000
Fair value of senior notes	102,258	[1]	103,068	[1]
4.00% Senior Notes due 2018
Debt Instrument [Line Items]
Carrying value of senior notes	50,000		50,000
Fair value of senior notes	51,267	[1]	52,476	[1]
5.25% Senior Notes due 2019
Debt Instrument [Line Items]
Carrying value of senior notes	175,000		175,000
Fair value of senior notes	188,392	[1]	195,584	[1]
4.64% Senior Notes due 2021
Debt Instrument [Line Items]
Carrying value of senior notes	100,000		100,000
Fair value of senior notes	$ 102,879	[1]	$ 107,418	[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.