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Stockholders' Equity (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Jun. 30, 2017
Dec. 31, 2018
Dec. 31, 2019
Jan. 21, 2015
Class of Stock [Line Items]              
Stock Repurchased and Retired During Period, Value   $ (40.0)   $ (40.0) $ (37.5)    
Number of shares authorized to be repurchased under the plan             20,000,000.0
UpfrontPaymentUnderAcceleratedStockRepurchaseProgram   $ 50.0   $ 50.0      
Stock Repurchased and Retired During Period, Shares (700,000) (3,100,000) (800,000) (3,200,000)      
UnsettledForwardContractIndexedtoIssuersStockClassifiedwithinStock   $ 10.0   $ 10.0      
Remaining shares authorized to be repurchased           3,600,000