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Goodwill and Intangible Assets - Amortization (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Expected amortization expense for future periods    
2018 $ 0.6  
2019 0.6  
2020 0.6  
2021 0.6  
2022 0.6  
Thereafter 7.8  
Intangible assets, net $ 10.8 $ 9.2