Fair Value Measurements (Fair Value of Senior Notes) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Senior Notes due 2021 at 4.875% [Member]
Carrying amount of senior notes	$ 497,173		$ 496,824
Senior Notes due 2021 at 4.875% [Member] | Fair value, inputs, level 1 [Member]
Estimated fair value of senior notes			533,150	[1]
Senior Notes due 2021 at 4.875% [Member] | Fair value inputs, level 2 [Member]
Estimated fair value of senior notes	559,870	[1]
Senior Notes due 2021 at 4.625% [Member]
Carrying amount of senior notes	299,545		299,493
Senior Notes due 2021 at 4.625% [Member] | Fair value, inputs, level 1 [Member]
Estimated fair value of senior notes			313,830	[1]
Senior Notes due 2021 at 4.625% [Member] | Fair value inputs, level 2 [Member]
Estimated fair value of senior notes	331,224	[1]
Senior Notes due 2022 at 3.800% [Member]
Carrying amount of senior notes	298,916		0
Senior Notes due 2022 at 3.800% [Member] | Fair value, inputs, level 1 [Member]
Estimated fair value of senior notes			0	[1]
Senior Notes due 2022 at 3.800% [Member] | Fair value inputs, level 2 [Member]
Estimated fair value of senior notes	$ 313,890	[1]

[1] Transferred from Level 1, as of December 31, 2011, to Level 2, as of December 31, 2012, within the hierarchy due to the absence of unadjusted, quoted prices in active markets.