Debt (Details) - USD ($)		1 Months Ended		12 Months Ended
	Jan. 15, 2016	Jan. 31, 2016	Nov. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Oct. 31, 2014
Debt Instrument [Line Items]
Commercial paper borrowings outstanding				$ 90,000,000	$ 0
Principal amount				2,951,822,000	3,226,943,000
Less: unamortized discount				5,288,000	6,653,000
Plus: unamortized interest rate swap proceeds				22,463,000	31,716,000
Total debt				2,968,997,000	3,252,006,000
Less: current portion long-term debt				461,085,000	324,879,000
Long-term debt				2,507,912,000	2,927,127,000
Repayments of loans				516,070,000	599,850,000	$ 1,079,207,000
Maximum borrowing capacity				1,000,000,000
Drawn on credit facility				0
Maturities of Long-term Debt [Abstract]
2016				461,085,000
2017				535,277,000
2018				600,168,000
2019				300,168,000
2020				412,000
Thereafter				$ 1,054,712,000
Eurodollar
Debt Instrument [Line Items]
Variable interest rate				0.90%
Commercial Paper
Debt Instrument [Line Items]
Interest rate				1.10%
Notes due 2015
Debt Instrument [Line Items]
Interest rate				5.00%
Debt face amount				$ 0	274,879,000
Repayments of loans				$ 275,000,000
Notes due 2016
Debt Instrument [Line Items]
Interest rate				4.75%
Debt face amount				$ 370,914,000	370,914,000
Notes due 2017
Debt Instrument [Line Items]
Interest rate				5.75%
Debt face amount				$ 385,109,000	385,109,000
Notes due March 2018
Debt Instrument [Line Items]
Interest rate				5.60%
Debt face amount				$ 250,000,000	250,000,000
Notes due May 2018
Debt Instrument [Line Items]
Interest rate				4.75%
Debt face amount				$ 350,000,000	350,000,000
Notes due 2019
Debt Instrument [Line Items]
Interest rate				6.25%
Debt face amount				$ 300,000,000	300,000,000
Notes due 2022
Debt Instrument [Line Items]
Interest rate			5.25%	5.25%
Debt face amount				$ 0	110,000,000
Repayments of loans			$ 110,000,000
Notes due 2024
Debt Instrument [Line Items]
Interest rate				4.625%
Debt face amount				$ 500,000,000	500,000,000
Development Loan
Debt Instrument [Line Items]
Debt face amount							$ 15,000,000
Jobs-Training Grant
Debt Instrument [Line Items]
Debt face amount							$ 1,000,000
Notes due 2037
Debt Instrument [Line Items]
Interest rate				5.25%
Debt face amount				$ 115,041,000	115,041,000
Notes due 2043
Debt Instrument [Line Items]
Interest rate				6.70%
Debt face amount				$ 425,000,000	425,000,000
Term loans
Debt Instrument [Line Items]
Debt face amount				$ 150,000,000	130,000,000
Repayments of loans					130,000,000
Weighted average interest rate				1.40%
Other Debt Obligations
Debt Instrument [Line Items]
Debt face amount				$ 15,758,000	$ 16,000,000
Subsequent Event | Notes due 2016
Debt Instrument [Line Items]
Interest rate	4.75%
Repayments of loans	$ 371,000,000
Subsequent Event | Notes due 2020
Debt Instrument [Line Items]
Debt face amount		$ 300,000,000
Subsequent Event | Notes due 2020 | Eurodollar
Debt Instrument [Line Items]
Variable interest rate		1.25%