Fair Value of Financial Instruments (Valuation Techniques Used to Measure the Fair Value of the Plan's Level 3 Investments) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($)
Comm Drive, LLC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 8
Capitalization rate	7.75%
CTE Drive, LLC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 6
Capitalization rate	9.00%
Oakbrook Parkway LLC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 24
Capitalization rate	8.00%
West Jefferson, LLC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 25
Capitalization rate	8.50%
MacCorkle Ave SE, LLC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 13
Capitalization rate	8.25%
S Pike Road West, LLC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 1
Capitalization rate	8.75%
N US 131, LLC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 1
Capitalization rate	9.50%
E Stockton Blvd, LLC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 5
Capitalization rate	7.75%