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Investment in CyrusOne (Details) - USD ($)
shares in Millions, $ in Millions
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Jan. 24, 2013
Investment [Line Items]        
Equity Method Investment, Ownership Percentage       69.00%
Investment owned in CyrusOne LP, Balance, Partnership Units     6.3 42.6
Dividends received from CyrusOne $ 2.1 $ 6.0    
CyrusOne [Member]        
Investment [Line Items]        
Investment Owned, Balance, Shares 6.9     1.9