Fair Value Measurements (Details 2) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Fair Value of Financial Instruments
Carrying Value	$ 3,757,716	$ 4,355,049
Fair Value	3,939,666	4,414,090
2.35% senior notes due September 2016
Fair Value of Financial Instruments
Carrying Value	349,921	349,887
Fair Value	$ 351,964	346,980
Interest rate on senior notes due (as a percent)	2.35%
6.15% senior notes due February 2018
Fair Value of Financial Instruments
Carrying Value	$ 931,307	930,693
Fair Value	$ 1,007,645	991,920
Interest rate on senior notes due (as a percent)	6.15%
9.25% senior notes due January 2019
Fair Value of Financial Instruments
Carrying Value	$ 339,607	339,607
Fair Value	$ 411,852	403,531
Interest rate on senior notes due (as a percent)	9.25%
5.00% senior notes due September 2020
Fair Value of Financial Instruments
Carrying Value	$ 698,406	698,253
Fair Value	$ 727,181	687,953
Interest rate on senior notes due (as a percent)	5.00%
4.625% senior notes due September 2021
Fair Value of Financial Instruments
Carrying Value	$ 698,508	698,388
Fair Value	$ 699,923	661,619
Interest rate on senior notes due (as a percent)	4.625%
5.10% senior notes due September 2023
Fair Value of Financial Instruments
Carrying Value	$ 348,957	348,893
Fair Value	$ 350,091	332,759
Interest rate on senior notes due (as a percent)	5.10%
Revolving Credit Facility
Fair Value of Financial Instruments
Carrying Value		450,000
Fair Value		450,000
Commercial paper
Fair Value of Financial Instruments
Carrying Value	$ 324,652	533,119
Fair Value	324,652	533,119
Other
Fair Value of Financial Instruments
Carrying Value	66,358	6,209
Fair Value	$ 66,358	$ 6,209