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7. Stockholders' Equity Treasury Shares (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Jun. 30, 2012
Dec. 31, 2017
Dec. 31, 2016
Equity, Class of Treasury Stock [Line Items]      
Stock Repurchase Program, Authorized Amount   $ 250.0 $ 18.0
Treasury Class-A [Member]      
Equity, Class of Treasury Stock [Line Items]      
Treasury Stock, Shares   582 122
Treasury Stock, Shares, Acquired 122 13,200  
Stock Issued During Period, Shares, Treasury Stock Reissued   12,740  
Treasury Class B [Member]      
Equity, Class of Treasury Stock [Line Items]      
Treasury Stock, Shares   917 917
Treasury Stock, Shares, Acquired 917