Borrowings (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Debt Disclosure [Abstract]
Interest expense	$ 33,053	$ 32,933	$ 66,058	$ 66,624
Interest income	(1,065)	(972)	(2,033)	(2,008)
Interest expense, net	31,988	$ 31,961	64,025	$ 64,616
Debt Instrument [Line Items]
Letters of Credit Outstanding, Amount	129,088		129,088
Short-term borrowings [Abstract]
Current installments	300,051		300,051		$ 299,956
Commercial Paper	81,900		81,900		478,000
Debt, Current	381,951		381,951		777,956
Long-term borrowings [Abstract]
Carrying amount of long-term debt	2,525,114		2,525,114		2,552,997
Long-term debt (noncurrent)	2,225,063		$ 2,225,063		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 June 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	$ 299,934		$ 299,934		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	$ 349,093		$ 349,093		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	$ 335,632		$ 335,632		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	$ 449,852		$ 449,852		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	$ 199,534		$ 199,534		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	$ 296,764		$ 296,764		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	$ 247,992		$ 247,992		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	$ 345,910		$ 345,910		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	$ 403		$ 403		$ 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