Fair Value Measurements (Fair Value of Senior Notes) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
$500 million, 4.875% Senior Notes due 2021 [Member]
Fair Value Measurements
Carrying amount of senior notes	$ 496,153	$ 495,951
$500 million, 4.875% Senior Notes due 2021 [Member] | Fair value, inputs, Level 2 [Member]
Fair Value Measurements
Estimated fair value of senior notes	545,237	542,078
$300 million, 4.625% Senior Notes due 2021 [Member]
Fair Value Measurements
Carrying amount of senior notes	298,466	298,396
$300 million, 4.625% Senior Notes due 2021 [Member] | Fair value, inputs, Level 2 [Member]
Fair Value Measurements
Estimated fair value of senior notes	325,027	319,620
$300 million, 3.800% Senior Notes due 2022 [Member]
Fair Value Measurements
Carrying amount of senior notes	297,617	297,535
$300 million, 3.800% Senior Notes due 2022 [Member] | Fair value, inputs, Level 2 [Member]
Fair Value Measurements
Estimated fair value of senior notes	315,934	303,595
$300 million, 3.850% Senior Notes due 2023 [Member]
Fair Value Measurements
Carrying amount of senior notes	298,190	298,136
$300 million, 3.850% Senior Notes due 2023 [Member] | Fair value, inputs, Level 2 [Member]
Fair Value Measurements
Estimated fair value of senior notes	314,925	$ 302,468
$500 million, 3.550% Senior Notes due 2026 [Member]
Fair Value Measurements
Carrying amount of senior notes	495,451
$500 million, 3.550% Senior Notes due 2026 [Member] | Fair value, inputs, Level 2 [Member]
Fair Value Measurements
Estimated fair value of senior notes	$ 517,907