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Stockholders' Equity - Narrative (Details)
3 Months Ended 12 Months Ended
Dec. 12, 2014
USD ($)
offering
shares
Oct. 23, 2014
May 02, 2010
$ / shares
shares
Mar. 31, 2016
USD ($)
$ / shares
Dec. 31, 2017
USD ($)
Vote / shares
shares
Dec. 31, 2016
USD ($)
shares
Dec. 31, 2015
USD ($)
shares
Jun. 30, 2015
USD ($)
Dec. 31, 2011
shares
Class of Stock [Line Items]                  
Convertible Series A Preferred Stock, shares authorized                 10,085
Common stock, shares, issued         51,136,000 50,513,000      
Tax impact on unrealized losses and gains on investment securities available-for-sale | $         $ 100,000 $ (200,000) $ (300,000)    
Payments for Repurchase of Common Stock | $         $ 51,969,000 59,013,000 $ 40,986,000    
December 2015 Repurchase Plan [Member]                  
Class of Stock [Line Items]                  
Stock Repurchased During Period, Value | $       $ 600,000          
Payments for Repurchase of Common Stock | $           $ 10,000,000      
Stock Repurchased During Period, Average Cost per Share | $ / shares       $ 16.15          
Common Class B [Member]                  
Class of Stock [Line Items]                  
Conversion of Stock, Shares Converted Class B to Series A Preferred                 6,859,000
Nonredeemable Convertible Preferred Stock [Member]                  
Class of Stock [Line Items]                  
Convertible Series A Preferred Stock, shares issued                 6,859
Common Class A [Member]                  
Class of Stock [Line Items]                  
Votes per share | Vote / shares         1        
Voting Threshold         24.90%        
Reduced Voting Power         14.90%        
Stock Repurchase Program, Authorized Amount | $               $ 150,000,000  
Treasury Stock, Value | $         $ 150,000,000        
Common Class A [Member] | Walmart [Member]                  
Class of Stock [Line Items]                  
Stock Repurchase Program, Period in Force     5 years            
Common stock, shares, issued     2,208,552   2,208,552        
Walmart Repurchase Price | $ / shares     $ 0.01            
Shares, monthly vesting amount     36,810   36,810        
Common Class A [Member] | TPG [Member]                  
Class of Stock [Line Items]                  
Period to file registration statement   75 days              
Remaining registrable securities (in shares) 1,840,001                
Aggregate number of shares issued in acquisition (percent) 30.00%                
Period after acquisition close 30 months                
Aggregate registered securities holdings | $ $ 30,000,000                
Number of underwritten offerings | offering 2                
Aggregate offering price | $ $ 30,000,000                
Common Class A [Member] | Common Stock [Member]                  
Class of Stock [Line Items]                  
Conversion of stock (in shares)           1,519,000      
Treasury stock acquired (in shares)         (1,326,000) (3,224,000) (1,956,000)