Borrowings (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Debt Disclosure [Abstract]
Interest expense	$ 33,098	$ 32,501	$ 99,156	$ 99,125
Interest income	(1,115)	(1,270)	(3,148)	(3,278)
Interest expense, net	31,983	$ 31,231	96,008	$ 95,847
Debt Instrument [Line Items]
Letters of Credit Outstanding, Amount	114,559		114,559
Short-term borrowings [Abstract]
Current installments	300,105		300,105		$ 299,956
Commercial Paper	161,200		161,200		478,000
Debt, Current	461,305		461,305		777,956
Long-term borrowings [Abstract]
Carrying amount of long-term debt	2,525,048		2,525,048		2,552,997
Long-term debt (noncurrent)	2,224,943		$ 2,224,943		2,253,041
Line of Credit Facility [Abstract]
Line of Credit Facility, Covenant Terms			Under the credit facility, the Company is required to maintain an interest coverage ratio of EBITDA to consolidated net interest expense of not less than 3.0 to 1.
Line of Credit Facility, Covenant Compliance			The Company was in compliance with this covenant and its other long-term debt covenants at September 30, 2015, and it expects to remain in compliance with all of its debt covenants.
Note due 2015 [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 299,984		$ 299,984		299,836
Debt instruments, maturity date			Oct. 15, 2015
Debt instrument, stated interest rate (in hundredths)	4.875%		4.875%
Term of debt instrument (in years)			10 years
Note due 2018 [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 349,175		$ 349,175		348,928
Debt instruments, maturity date			Mar. 15, 2018
Debt instrument, stated interest rate (in hundredths)	5.45%		5.45%
Term of debt instrument (in years)			10 years
Note due 2020 [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 335,355		$ 335,355		363,970
Debt instruments, maturity date			Dec. 01, 2020
Debt instrument, stated interest rate (in hundredths)	2.125%		2.125%
Term of debt instrument (in years)			7 years
Note due 2021 [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 449,859		$ 449,859		449,839
Debt instruments, maturity date			Mar. 01, 2021
Debt instrument, stated interest rate (in hundredths)	4.30%		4.30%
Term of debt instrument (in years)			10 years
Debentures due 2028 [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 199,543		$ 199,543		199,517
Debt instruments, maturity date			Jun. 01, 2028
Debt instrument, stated interest rate (in hundredths)	6.65%		6.65%
Term of debt instrument (in years)			30 years
Debenture due 2035 [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 296,804		$ 296,804		296,685
Debt instruments, maturity date			Oct. 15, 2035
Debt instrument, stated interest rate (in hundredths)	5.375%		5.375%
Term of debt instrument (in years)			30 years
Note due 2038 [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 248,014		$ 248,014		247,948
Debt instruments, maturity date			Mar. 15, 2038
Debt instrument, stated interest rate (in hundredths)	6.60%		6.60%
Term of debt instrument (in years)			30 years
Note due 2041 [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 345,949		$ 345,949		345,830
Debt instruments, maturity date			Mar. 01, 2041
Debt instrument, stated interest rate (in hundredths)	5.375%		5.375%
Term of debt instrument (in years)			30 years
Other long term debt instruments [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 365		$ 365		$ 444
Credit Agreement [Member]
Line of Credit Facility [Abstract]
Line of Credit Facility, Expiration Date			Nov. 10, 2016
Unsecured revolving credit facility, maximum borrowing capacity	$ 1,000,000		$ 1,000,000
Minimum [Member]
Line of Credit Facility [Abstract]
Line of Credit Facility, Expiration Date			Dec. 31, 2015
Maximum [Member]
Line of Credit Facility [Abstract]
Line of Credit Facility, Expiration Date			Dec. 31, 2020