Debt (Details) (USD $)																	3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 2.35% senior notes due September 2016	Dec. 31, 2013 2.35% senior notes due September 2016	Mar. 31, 2014 6.15% senior notes due February 2018	Dec. 31, 2013 6.15% senior notes due February 2018	Mar. 31, 2014 9.25% senior notes due January 2019	Dec. 31, 2013 9.25% senior notes due January 2019	Mar. 31, 2014 5.00% senior notes due September 2020	Dec. 31, 2013 5.00% senior notes due September 2020	Mar. 31, 2014 4.625% senior notes due September 2021	Dec. 31, 2013 4.625% senior notes due September 2021	Mar. 31, 2014 5.10% senior notes due September 2023	Dec. 31, 2013 5.10% senior notes due September 2023	Mar. 31, 2014 Revolving Credit Facility	Dec. 31, 2013 Revolving Credit Facility	Mar. 31, 2014 Revolving Credit Facility Minimum	Mar. 31, 2014 Commercial paper	Dec. 31, 2013 Commercial paper
Long-term debt
Senior Notes			$ 349,836,000	$ 349,820,000	$ 970,234,000	$ 969,928,000	$ 339,607,000	$ 339,607,000	$ 698,208,000	$ 697,947,000	$ 698,024,000	$ 698,148,000	$ 348,797,000	$ 348,765,000
Revolving credit facility															117,500,000	170,000,000
Other	5,316,000	10,243,000
Long-term Debt	3,817,772,000	3,914,302,000																290,250,000	329,844,000
Less: Current portion	5,296,000	10,185,000
Long-term debt	3,812,476,000	3,904,117,000
Interest rate on senior notes due (as a percent)			2.35%		6.15%		9.25%		5.00%		4.625%		5.10%
Aggregate amount of senior notes																		290,300,000
Weighted average interest rate on revolving credit facilities (as a percent)															1.46%			0.33%
Remaining period to maturity																	1 year
Remaining availability under credit facility															$ 117,500,000