Fair Value Measurements (Fair Value of Senior Notes) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Senior Notes due 2021 at 4.875% [Member]
Estimated fair value of senior notes	$ 524,434	$ 559,870
Senior Notes due 2021 at 4.625% [Member]
Estimated fair value of senior notes	310,141	331,224
Senior Notes due 2022 at 3.800% [Member]
Estimated fair value of senior notes	290,453	313,890
Senior Notes due 2023 at 3.850% [Member]
Estimated fair value of senior notes	289,362	0
Senior Notes due 2021 at 4.875% [Member]
Carrying amount of senior notes	497,525	497,173
Senior Notes due 2021 at 4.625% [Member]
Carrying amount of senior notes	299,598	299,545
Senior Notes due 2022 at 3.800% [Member]
Carrying amount of senior notes	299,011	298,916
Senior Notes due 2023 at 3.850% [Member]
Carrying amount of senior notes	$ 299,976	$ 0