Fair Value Measurements (Fair Value of Senior Notes) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
$500 million, 4.875% Senior Notes due 2021 [Member]
Fair Value Measurements
Carrying amount of senior notes	$ 495,951	$ 495,144	[1]
$500 million, 4.875% Senior Notes due 2021 [Member] | Fair value, inputs, Level 2 [Member]
Fair Value Measurements
Estimated fair value of senior notes	542,078	566,700
$300 million, 4.625% Senior Notes due 2021 [Member]
Fair Value Measurements
Carrying amount of senior notes	298,396	298,113	[1]
$300 million, 4.625% Senior Notes due 2021 [Member] | Fair value, inputs, Level 2 [Member]
Fair Value Measurements
Estimated fair value of senior notes	319,620	337,222
$300 million, 3.800% Senior Notes due 2022 [Member]
Fair Value Measurements
Carrying amount of senior notes	297,535	297,215	[1]
$300 million, 3.800% Senior Notes due 2022 [Member] | Fair value, inputs, Level 2 [Member]
Fair Value Measurements
Estimated fair value of senior notes	303,595	310,749
$300 million, 3.850% Senior Notes due 2023 [Member]
Fair Value Measurements
Carrying amount of senior notes	298,136	297,925	[1]
$300 million, 3.850% Senior Notes due 2023 [Member] | Fair value, inputs, Level 2 [Member]
Fair Value Measurements
Estimated fair value of senior notes	$ 302,468	$ 311,656

[1] Prior period amounts have been reclassified to conform to current period presentation, due to the Company's adoption of new accounting standards during the fourth quarter ended December 31, 2015, see Note 1 for further information.