Borrowings (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Interest expense and interest income [Abstract]
Interest expense	$ 30,872,000	$ 31,638,000
Interest income	(628,000)	(1,607,000)
Interest expense, net	30,244,000	30,031,000
Short-term borrowings [Abstract]
Current installments	4,198,000		3,266,000
Commercial Paper	698,400,000		607,500,000
Debt, Current	702,598,000		610,766,000
Long-term borrowings [Abstract]
Carrying amount of long-term debt	2,193,719,000		2,192,616,000
Long-term debt (noncurrent)	2,189,521,000		2,189,350,000
Note due 2015 [Member]
Long-term borrowings [Abstract]
Long-term debt	299,490,000		299,441,000
Debt instruments, maturity date	Oct. 15, 2015
Debt instrument, stated interest rate (in hundredths)	4.875%
Term of debt instrument (in years)	10 years
Note due 2018 [Member]
Long-term borrowings [Abstract]
Long-term debt	348,350,000		348,268,000
Debt instruments, maturity date	Mar. 15, 2018
Debt instrument, stated interest rate (in hundredths)	5.45%
Term of debt instrument (in years)	10 years
Note due 2021 [Member]
Long-term borrowings [Abstract]
Long-term debt	449,793,000		449,787,000
Debt instruments, maturity date	Mar. 01, 2021
Debt instrument, stated interest rate (in hundredths)	4.30%
Term of debt instrument (in years)	10 years
Note due 2038 [Member]
Long-term borrowings [Abstract]
Long-term debt	247,793,000		247,771,000
Debt instruments, maturity date	Mar. 15, 2038
Debt instrument, stated interest rate (in hundredths)	6.60%
Term of debt instrument (in years)	30 years
Note due 2041 [Member]
Long-term borrowings [Abstract]
Long-term debt	345,551,000		345,511,000
Debt instruments, maturity date	Mar. 01, 2041
Debt instrument, stated interest rate (in hundredths)	5.375%
Term of debt instrument (in years)	30 years
Debentures due 2028 [Member]
Long-term borrowings [Abstract]
Long-term debt	199,457,000		199,448,000
Debt instruments, maturity date	Jun. 01, 2028
Debt instrument, stated interest rate (in hundredths)	6.65%
Term of debt instrument (in years)	30 years
Debenture due 2035 [Member]
Long-term borrowings [Abstract]
Long-term debt	296,406,000		296,367,000
Debt instruments, maturity date	Oct. 15, 2035
Debt instrument, stated interest rate (in hundredths)	5.375%
Term of debt instrument (in years)	30 years
Other long term debt instruments [Member]
Long-term borrowings [Abstract]
Long-term debt	6,879,000		6,023,000
Credit Agreement [Member]
Line of Credit Facility [Abstract]
Line of Credit Facility, Expiration Date	Nov. 10, 2016
Unsecured revolving credit facility, maximum borrowing capacity	$ 1,000,000,000