Debt (Narrative) (Details) (Credit Facility [Member], USD $)	3 Months Ended		3 Months Ended
	Mar. 31, 2014 item	Dec. 16, 2013	Mar. 31, 2014 Over LIBOR [Member]	Mar. 31, 2013 Over LIBOR [Member]
Debt Instrument [Line Items]
Unsecured revolving credit facility, borrowing capacity		$ 2,000,000,000
Unsecured revolving credit facility, maturity date	Dec. 01, 2018
Number of extension options	2
Extension term	1 year
Unsecured revolving credit facility, increase in current borrowing capacity potential		$ 500,000,000
Basis points			1.50%	2.00%
Debt percentage of adjusted book capital structure covenant		60.00%