Debt (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013 2.35% senior notes due September 2016	Sep. 30, 2013 2.35% senior notes due September 2016	Dec. 31, 2013 6.15% senior notes due February 2018	Dec. 31, 2012 6.15% senior notes due February 2018	Dec. 31, 2013 9.25% senior notes due January 2019	Dec. 31, 2012 9.25% senior notes due January 2019	Dec. 31, 2013 5.00% senior notes due September 2020	Dec. 31, 2012 5.00% senior notes due September 2020	Dec. 31, 2013 4.625% senior notes due September 2021	Dec. 31, 2012 4.625% senior notes due September 2021	Dec. 31, 2013 5.10% senior notes due September 2023	Sep. 30, 2013 5.10% senior notes due September 2023	Dec. 31, 2013 Revolving Credit Facility	Dec. 31, 2012 Revolving Credit Facility	Dec. 31, 2013 Commercial paper
Long-term debt
Senior Notes			$ 349,820		$ 969,928	$ 968,708	$ 339,607	$ 1,125,000	$ 697,947	$ 697,648	$ 698,148	$ 697,907	$ 348,765
Revolving credit facility															170,000	890,000
Other	10,243	437
Long-term Debt	3,914,302	4,379,700															329,844
Current debt	10,185	364
Long-term debt	$ 3,904,117	$ 4,379,336
Interest rate on senior notes due (as a percent)			2.35%	2.35%	6.15%		9.25%		5.00%		4.625%		5.10%	5.10%