Credit Agreement (Details)	3 Months Ended
	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Line of Credit Facility [Line Items]
Actual net worth	$ 10,525,000,000	$ 10,346,000,000
Credit Agreement | Line of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Term	5 years
Maximum borrowing capacity	$ 1,000,000,000
Minimum level of net worth required	8,598,000,000
Actual net worth	$ 10,500,000,000
Required maximum leverage ratio	3.0
Actual leverage ratio	1.5
Uncommitted incremental loan facility	$ 250,000,000
Line of credit, outstanding borrowings	0
Remaining borrowing capacity	999,000,000
Credit Agreement | Line of Credit | Letter of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 1,000,000
Credit Agreement | London Interbank Offered Rate (LIBOR) | Line of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Interest rate spread	1.10%
Facility fee (percentage)	0.15%
Credit Agreement | Minimum | London Interbank Offered Rate (LIBOR) | Line of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Interest rate spread	0.90%
Facility fee (percentage)	0.10%
Credit Agreement | Maximum | London Interbank Offered Rate (LIBOR) | Line of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Interest rate spread	1.50%
Facility fee (percentage)	0.25%