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Derivatives - Interest Rate Swaps (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
May 31, 2017
Derivatives      
Accumulated other comprehensive loss $ (2.0) $ (2.0) $ (2.2)
Interest income (expense) expected to be reclassified out of accumulated other comprehensive income (loss) over the next twelve months $ (0.1)    
5.45% Senior unsecured notes due 2047      
Derivatives      
Stated interest rate 5.45%