Fair Value Measurements (Fair Value of Senior Notes) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
4.875% Senior Notes due 2021 [Member]
Fair Value Measurements
Carrying amount of senior notes	$ 498,140	$ 497,876
4.875% Senior Notes due 2021 [Member] | Fair value, inputs, Level 2 [Member]
Fair Value Measurements
Estimated fair value of senior notes	550,638	566,700
4.625% Senior Notes due 2021 [Member]
Fair Value Measurements
Carrying amount of senior notes	299,689	299,650
4.625% Senior Notes due 2021 [Member] | Fair value, inputs, Level 2 [Member]
Fair Value Measurements
Estimated fair value of senior notes	324,469	337,222
3.800% Senior Notes due 2022 [Member]
Fair Value Measurements
Carrying amount of senior notes	299,186	299,109
3.800% Senior Notes due 2022 [Member] | Fair value, inputs, Level 2 [Member]
Fair Value Measurements
Estimated fair value of senior notes	307,285	310,749
3.850% Senior Notes due 2023 [Member]
Fair Value Measurements
Carrying amount of senior notes	299,980	299,980
3.850% Senior Notes due 2023 [Member] | Fair value, inputs, Level 2 [Member]
Fair Value Measurements
Estimated fair value of senior notes	$ 301,736	$ 311,656