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Stock-Based Compensation (Detail) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended
Jan. 01, 2018
Mar. 31, 2018
Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares available for grant (in shares)   5,600,000.0  
Stock-based compensation expense (in USD)   $ 8.0 $ 4.1
Stock-based compensation amortization expense unrecognized (in USD)   $ 27.5  
Average remaining vesting period (in years)   2 years 4 months 24 days  
Employee Stock Option      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock options acquired (in shares) 1,800,000    
Restricted Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number acquired outstanding stock awards (in shares) 336,000    
Time-based RSUs aquired that remained outstanding (in shares)   199,000  
Number of shares granted during the period (in shares)   450,000 371,000
Award vesting period (in years)   3 years  
Average grant date fair value (usd per share)   $ 12.35 $ 11.05
Restricted Stock Units | Growth in Tangible Book Value per Share Plus Dividends      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting rights as a percentage   75.00%  
Restricted Stock Units | Total Shareholder Return as Compared to Our Peer Group      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting rights as a percentage   25.00%  
Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares granted during the period (in shares)   1,100,000.0 393,000
Award vesting period (in years)   3 years  
Average grant date fair value (usd per share)   $ 11.81 $ 11.66
Award vesting rights as a percentage   33.33%  
2016 Long-Term Stock Incentive Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of acquired outstanding stock awards that remain outstanding (in shares)   1,200,000  
Weighted average exercise price (in USD per share)   $ 4.97