Borrowings (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Interest expense and interest income [Abstract]
Interest expense	$ 31,478,000	$ 31,080,000	$ 63,123,000	$ 62,116,000
Interest income	(1,761,000)	(2,923,000)	(3,379,000)	(5,641,000)
Interest expense, net	29,717,000	28,157,000	59,744,000	56,475,000
Debt Instrument [Line Items]
Carrying amount	2,195,152,000		2,195,152,000		2,187,252,000
Current installments	(3,402,000)		(3,402,000)		(1,022,000)
Long-term debt	2,191,750,000		2,191,750,000		2,186,230,000
Note due 2015 [Member]
Debt Instrument [Line Items]
Debt instrument, stated interest rate (in hundredths)	4.875%		4.875%
Term of debt instrument (in years)			10 years
Debt instruments, maturity date			Oct. 15, 2015
Carrying amount	299,342,000		299,342,000		299,244,000
Note due 2018 [Member]
Debt Instrument [Line Items]
Debt instrument, stated interest rate (in hundredths)	5.45%		5.45%
Term of debt instrument (in years)			10 years
Debt instruments, maturity date			Mar. 15, 2018
Carrying amount	348,103,000		348,103,000		347,938,000
Note due 2021 [Member]
Debt Instrument [Line Items]
Debt instrument, stated interest rate (in hundredths)	4.30%		4.30%
Term of debt instrument (in years)			10 years
Debt instruments, maturity date			Mar. 01, 2021
Carrying amount	449,774,000		449,774,000		449,761,000
Note due 2038 [Member]
Debt Instrument [Line Items]
Debt instrument, stated interest rate (in hundredths)	6.60%		6.60%
Term of debt instrument (in years)			30 years
Debt instruments, maturity date			Mar. 15, 2038
Carrying amount	247,727,000		247,727,000		247,683,000
Note due 2041 [Member]
Debt Instrument [Line Items]
Debt instrument, stated interest rate (in hundredths)	5.375%		5.375%
Term of debt instrument (in years)			30 years
Debt instruments, maturity date			Mar. 01, 2041
Carrying amount	345,432,000		345,432,000		345,352,000
Debentures due 2028 [Member]
Debt Instrument [Line Items]
Debt instrument, stated interest rate (in hundredths)	6.65%		6.65%
Term of debt instrument (in years)			30 years
Debt instruments, maturity date			Jun. 01, 2028
Carrying amount	199,431,000		199,431,000		199,414,000
Debenture due 2035 [Member]
Debt Instrument [Line Items]
Debt instrument, stated interest rate (in hundredths)	5.375%		5.375%
Term of debt instrument (in years)			30 years
Debt instruments, maturity date			Oct. 15, 2035
Carrying amount	296,287,000		296,287,000		296,208,000
Other long term debt instruments [Member]
Debt Instrument [Line Items]
Carrying amount	9,056,000		9,056,000		1,652,000
Credit Agreement [Member]
Debt Instrument [Line Items]
Unsecured revolving credit facility, maximum borrowing capacity	$ 1,000,000,000		$ 1,000,000,000
Line of Credit Facility, Expiration Date			Nov. 10, 2016