Fair Value Measurements (Fair Value of Senior Notes) (Details) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015
$500 million, 4.875% Senior Notes due 2021 [Member]
Fair Value Measurements
Carrying amount of senior notes	[1]	$ 496,758	$ 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		538,678	542,078
$300 million, 4.625% Senior Notes due 2021 [Member]
Fair Value Measurements
Carrying amount of senior notes	[2]	298,679	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		321,633	319,620
$300 million, 3.800% Senior Notes due 2022 [Member]
Fair Value Measurements
Carrying amount of senior notes	[3]	297,868	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		310,802	303,595
$300 million, 3.850% Senior Notes due 2023 [Member]
Fair Value Measurements
Carrying amount of senior notes	[4]	298,355	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		307,860	$ 302,468
$500 million, 3.550% Senior Notes due 2026 [Member]
Fair Value Measurements
Carrying amount of senior notes	[5]	495,359
$500 million, 3.550% Senior Notes due 2026 [Member] | Fair value, inputs, Level 2 [Member]
Fair Value Measurements
Estimated fair value of senior notes		$ 498,537

[1] Net of unamortized discount of $1.4 million and $1.8 million as of December 31, 2016 and 2015, respectively, and debt issuance costs of $1.8 million and $2.3 million as of December 31, 2016 and 2015, respectively.

[2] Net of unamortized discount of $0.2 million and $0.3 million as of December 31, 2016 and 2015, respectively, and debt issuance costs of $1.1 million and $1.3 million as of December 31, 2016 and 2015, respectively.

[3] Net of unamortized discount of $0.7 million and $0.8 million as of December 31, 2016 and 2015, respectively, and debt issuance costs of $1.5 million and $1.7 million as of December 31, 2016 and 2015, respectively.

[4] Net of unamortized discount of less than $0.1 million as of December 31, 2016 and 2015, and debt issuance costs of $1.6 million and $1.8 million as of December 31, 2016 and 2015, respectively.

[5] Net of unamortized discount of $0.8 million as of December 31, 2016, and debt issuance costs of $3.9 million as of December 31, 2016.