Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Carrying values and fair values of the Company's debt instruments
Carrying value of senior notes	$ 887,792	$ 1,178,657
6.22% Senior Notes due 2015 [Member]
Carrying values and fair values of the Company's debt instruments
Carrying value of senior notes	175,000	175,000
Fair value of senior notes	184,961	186,305
3.30% Senior Notes due 2016 [Member]
Carrying values and fair values of the Company's debt instruments
Carrying value of senior notes	100,000	100,000
Fair value of senior notes	100,402	98,980
4.00% Senior Notes due 2018 [Member]
Carrying values and fair values of the Company's debt instruments
Carrying value of senior notes	50,000	50,000
Fair value of senior notes	50,783	51,220
5.25% Senior Notes due 2019 [Member]
Carrying values and fair values of the Company's debt instruments
Carrying value of senior notes	175,000	175,000
Fair value of senior notes	172,237	174,125
4.64% Senior Notes due 2021 [Member]
Carrying values and fair values of the Company's debt instruments
Carrying value of senior notes	100,000	100,000
Fair value of senior notes	$ 102,889	$ 104,250