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Securities - Schedule of Amortized Cost and Fair Value of Securities Held to Maturity (Detail) - USD ($)
$ in Thousands
Jun. 30, 2016
Dec. 31, 2015
Investment Securities Held To Maturity [Line Items]    
Securities Held To Maturity, Amortized Cost $ 2,064,305 $ 1,637,061
Securities Held To Maturity, Gross Unrealized Gains 41,867 15,943
Securities Held To Maturity, Gross Unrealized Losses (1,390) (9,588)
Securities held to maturity, fair value 2,104,782 1,643,416
U.S. Treasury [Member]    
Investment Securities Held To Maturity [Line Items]    
Securities Held To Maturity, Amortized Cost 500 500
Securities Held To Maturity, Gross Unrealized Gains 204 153
Securities held to maturity, fair value 704 653
U.S. Government-Sponsored Entities [Member]    
Investment Securities Held To Maturity [Line Items]    
Securities Held To Maturity, Amortized Cost 238,464 137,385
Securities Held To Maturity, Gross Unrealized Gains 2,073 809
Securities Held To Maturity, Gross Unrealized Losses   (395)
Securities held to maturity, fair value 240,537 137,799
Agency Mortgage-Backed Securities [Member]    
Investment Securities Held To Maturity [Line Items]    
Securities Held To Maturity, Amortized Cost 882,467 709,970
Securities Held To Maturity, Gross Unrealized Gains 22,422 9,858
Securities Held To Maturity, Gross Unrealized Losses   (1,176)
Securities held to maturity, fair value 904,889 718,652
Agency Collateralized Mortgage Obligations [Member]    
Investment Securities Held To Maturity [Line Items]    
Securities Held To Maturity, Amortized Cost 614,480 499,694
Securities Held To Maturity, Gross Unrealized Gains 6,356 803
Securities Held To Maturity, Gross Unrealized Losses (1,305) (7,657)
Securities held to maturity, fair value 619,531 492,840
Non-Agency Collateralized Mortgage Obligations [Member]    
Investment Securities Held To Maturity [Line Items]    
Securities Held To Maturity, Amortized Cost 2,143 2,681
Securities Held To Maturity, Gross Unrealized Gains 5 14
Securities Held To Maturity, Gross Unrealized Losses (4)  
Securities held to maturity, fair value 2,144 2,695
Commercial Mortgage-Backed Securities [Member]    
Investment Securities Held To Maturity [Line Items]    
Securities Held To Maturity, Amortized Cost 50,294 51,258
Securities Held To Maturity, Gross Unrealized Gains 1,776 115
Securities Held To Maturity, Gross Unrealized Losses (81) (259)
Securities held to maturity, fair value 51,989 51,114
States of the U.S. and Political Subdivisions [Member]    
Investment Securities Held To Maturity [Line Items]    
Securities Held To Maturity, Amortized Cost 275,957 235,573
Securities Held To Maturity, Gross Unrealized Gains 9,031 4,191
Securities Held To Maturity, Gross Unrealized Losses   (101)
Securities held to maturity, fair value $ 284,988 $ 239,663