Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Carrying values and fair values of the Company's debt instruments
Carrying value of senior notes	$ 982,105	$ 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	196,605	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	102,799	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	52,501	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	192,868	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	$ 103,804	$ 104,250