Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies - Retirement Benefits (Details)

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Summary of Significant Accounting Policies - Retirement Benefits (Details) - USD ($)
$ in Thousands
6 Months Ended 12 Months Ended
Dec. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Jun. 30, 2015
Summary of Significant Accounting Policies        
Pension contributions $ 122 $ 1,264 $ 976 $ 240