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Subsequent Events - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Millions
2 Months Ended 5 Months Ended 12 Months Ended
Feb. 19, 2016
Feb. 19, 2016
Dec. 31, 2015
Subsequent Event [Line Items]      
Repurchase of common stock, shares     11,949,530
Subsequent Event [Member]      
Subsequent Event [Line Items]      
Repurchase of common stock, shares 11,100,000 23,100,000  
Proceeds from share repurchase program $ 102.7 $ 217.0  
Repurchase of common stock, price per share $ 9.23 $ 9.40  
Weighted average remaining maturity,FHLB Des Moines 4 years 4 months 24 days    
Subsequent Event [Member] | Maximum      
Subsequent Event [Line Items]      
Outstanding advances, FHLB Des Moines $ 3,800.0