Borrowings Under Credit Facilities (Details) $ in Millions	6 Months Ended
	Jun. 30, 2015 USD ($) item	Dec. 31, 2014 USD ($)
Line of Credit Facility [Line Items]
Secured revolving credit facility	$ 2,400
Number of revolving credit facilities extension options | item	2
Extension period for revolving line of credit	1 year
Additional increase in borrowing capacity amount	$ 100
Outstanding borrowing under the revolving credit facility	$ 780	$ 711
Outstanding borrowing, interest rate	1.31%	1.29%
Letters of credit	$ 7	$ 7
Commitment fee	0.125%
Borrowing capacity available under the credit facility	$ 1,613	$ 1,682
Maximum [Member]
Line of Credit Facility [Line Items]
Commitment rate reduction under the revolving credit facility	0.275%
Required debt-to-capitalization ratio	1.00
Minimum [Member]
Line of Credit Facility [Line Items]
Commitment rate reduction under the revolving credit facility	0.10%
Required debt-to-capitalization ratio	0.65
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Outstanding borrowing, interest rate	1.325%
London Interbank Offered Rate (LIBOR) [Member]
Line of Credit Facility [Line Items]
Spread over variable rate	1.125%
London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Spread over variable rate	1.75%
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Spread over variable rate	1.00%
Federal Funds Effective Rate [Member]
Line of Credit Facility [Line Items]
Spread over variable rate	0.50%
One Month LIBOR [Member]
Line of Credit Facility [Line Items]
Spread over variable rate	1.00%
One Month LIBOR [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Spread over variable rate	0.75%
One Month LIBOR [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Spread over variable rate	0.00%