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Commitments and Contingencies - Letters of Credit (Details) - USD ($)
$ in Millions
1 Months Ended
Feb. 28, 2018
Sep. 30, 2018
Dec. 31, 2017
Oct. 31, 2016
Interest-bearing Deposits        
Loss Contingencies [Line Items]        
Restricted cash   $ 1.0 $ 2.0  
Financial Standby Letter of Credit        
Loss Contingencies [Line Items]        
Letters of credit outstanding       $ 1.9
Required cash deposit with financial institution       $ 1.9
Required cash deposit with financial institution after February 2018 $ 1.0