Borrowings (Details) (USD $)	3 Months Ended		6 Months Ended			6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended			3 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Jun. 30, 2011 Note due 2011 [Member]	Dec. 31, 2010 Note due 2011 [Member]	Jun. 30, 2011 Note due 2015 [Member]	Dec. 31, 2010 Note due 2015 [Member]	Jun. 30, 2011 Note due 2018 [Member]	Dec. 31, 2010 Note due 2018 [Member]	Jun. 30, 2011 Note due 2021 [Member]	Dec. 31, 2010 Note due 2021 [Member]	Jun. 30, 2011 Note due 2038 [Member]	Dec. 31, 2010 Note due 2038 [Member]	Jun. 30, 2011 Note due 2041 [Member]	Dec. 31, 2010 Note due 2041 [Member]	Jun. 30, 2011 Debentures due 2028 [Member]	Dec. 31, 2010 Debentures due 2028 [Member]	Jun. 30, 2011 Debentures due 2035 [Member]	Dec. 31, 2010 Debentures due 2035 [Member]	Jun. 30, 2011 Other long term debt instruments [Member]	Dec. 31, 2010 Other long term debt instruments [Member]	Mar. 31, 2011 Total of debt instruments issued 2011 [Member]	Jun. 30, 2011 Commercial Paper [Member]	Dec. 31, 2010 Commercial Paper [Member]
Debt Instrument [Line Items]
Debt instrument, stated interest rate (in hundredths)						6.50%	6.50%	4.875%	4.875%	5.45%	5.45%	4.30%	4.30%	6.60%	6.60%	5.375%	5.375%	6.65%	6.65%	5.375%	5.375%
Term of debt instrument (in number of years)						10	10	10	10	10	10	10	10	30	30	30	30	30	30	30	30
Debt instrument, maturity date						Feb. 15, 2011	Feb. 15, 2011	Oct. 15, 2015	Oct. 15, 2015	Mar. 15, 2018	Mar. 15, 2018	Mar. 01, 2021	Mar. 01, 2021	Mar. 15, 2038	Mar. 15, 2038	Mar. 01, 2041	Mar. 01, 2041	Jun. 01, 2028	Jun. 01, 2028	Oct. 15, 2035	Oct. 15, 2035
Carrying amount	$ 2,187,986,000		$ 2,187,986,000		$ 1,792,811,000	$ 0	$ 399,986,000	$ 299,145,000	$ 299,047,000	$ 347,773,000	$ 347,608,000	$ 449,748,000	$ 0	$ 247,639,000	$ 247,595,000	$ 345,272,000	$ 0	$ 199,396,000	$ 199,379,000	$ 296,128,000	$ 296,048,000	$ 2,885,000	$ 3,148,000		$ 46,000,000	$ 15,000,000
Current installments	1,748,000		1,748,000		1,925,000
Total long-term debt	2,186,238,000		2,186,238,000		1,790,886,000
Issuing date of debt instrument												February 22, 2011				February 22, 2011
Face amount of debt instrument						400,000,000						450,000,000				350,000,000
Proceeds from the issuance of debt instruments																								788,971,000
Unsecured revolving credit facility, maximum borrowing capacity	1,000,000,000		1,000,000,000
Revolving credit facility expiration date	November 9, 2012
Interest expense, borrowings	31,120,000	28,996,000	62,180,000	57,842,000
Interest income	$ 2,986,000	$ 2,054,000	$ 5,760,000	$ 3,731,000