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Senior Bank Debt (Details) (USD $)
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2013
Line of Credit [Member]
Dec. 31, 2012
Line of Credit [Member]
Apr. 30, 2013
Senior unsecured revolving credit facility [Member]
Deb Instrument [Line Items]          
Maximum borrowing capacity         $ 1,250,000,000
Line of credit facility amount outstanding     525,000,000 325,000,000  
Weighted average interest rate 1.95% 2.77%      
Commitment fee amount $ 3,200,000 $ 3,700,000