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Stockholders' Equity - Nonqualified Cash Plan and Equity Incentive Plans (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Stockholders' Equity    
Deferred compensation investment $ 2.7 $ 2.2
2007 Plan and 2007 Director Plan    
Stockholders' Equity    
Award term P10Y  
Vesting percentage 100.00%  
2007 Plan and 2007 Director Plan | First Anniversary    
Stockholders' Equity    
Vesting percentage 25.00%  
Maximum | 2007 Plan and 2007 Director Plan | Options to purchase common stock    
Stockholders' Equity    
Number of shares reserved for issuance   9,457,741