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Summary of Significant Accounting Policies Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Accounting Policies [Abstract]    
Percentage ownership by company 90.00%  
Percentage ownership by TDR Capital LLP 10.00%  
Provisional tax benefit from remeasurement of deferred tax liabilities   $ 28.1
Provisional tax expense from the deemed repatriation of foreign earnings   $ 2.4
Reclassification from accumulated other comprehensive income to retained earnings from adoption of ASU 2018-02 $ 2.5