Short-Term Loans (Details) (USD $)	12 Months Ended
	Dec. 31, 2012 institution item	Dec. 31, 2011
Short-Term Loans
Revolving credit facility maximum borrowing capacity	$ 1,500,000,000
Number of times the maturity date of the credit facility can be extended by one year	2
Length of extension of line of credit	1 year
Aggregate potential increased lenders' commitment	2,000,000,000
Number of financial institutions underwriting credit facility of the entity	16
Commitment fee paid to maintain credit facility (as a percent)	0.25%	0.30%
Commitment fee paid by partner to maintain credit facility (as a percent)	0.25%
Maximum amount of outstanding short-term loans at any time during the period		104,000,000
Average daily balance of short-term loans outstanding during the period		$ 5,500,000
Weighted average annual interest rates of average daily balance of short-term loans (as a percent)		1.81%
Percentage of total debt to total capital, maximum	65.00%