Leases (Sale and Leaseback Agreement) (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended	1 Months Ended
	May 03, 2013 Building acre	Jul. 26, 2013 Building
May 2013 Sales Lease Back Agreement
Sale Leaseback Transaction [Line Items]
Area of land	18
Number of real estate properties	4
Period to complete transaction	6 years
Proceeds received upon closure of first phase of the transaction		$ 83
Number of buildings leased back		3
Number of buildings sold		2
Fairfax County, Virginia, Two Office Buildings
Sale Leaseback Transaction [Line Items]
Proceeds received upon closure of first phase of the transaction		40
Book value of the building sold		42
Net loss on sales and disposals of assets		2
Term of lease		P6M
Notes Payable, Other Payables
Sale Leaseback Transaction [Line Items]
Term of debt instrument		7 years
Rate of interest		3.70%
Other Long-term Liabilities | Multi-level Parking Garage
Sale Leaseback Transaction [Line Items]
Allocated consideration to Note parable and Other long-term liabilities		1
Other Long-term Liabilities | Surface Parking Lots
Sale Leaseback Transaction [Line Items]
Allocated consideration to Note parable and Other long-term liabilities		4
Notes payable and long-term debt, net of current portion | Notes Payable, Other Payables
Sale Leaseback Transaction [Line Items]
Allocated consideration to Note parable and Other long-term liabilities		$ 38