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Securities - Schedule of Amortized Cost and Fair Value of Securities Held to Maturity (Detail) - USD ($)
$ in Thousands
Sep. 30, 2016
Dec. 31, 2015
Investment Securities Held To Maturity [Line Items]    
Securities Held To Maturity, Amortized Cost $ 2,249,245 $ 1,637,061
Securities Held To Maturity, Gross Unrealized Gains 33,576 15,943
Securities Held To Maturity, Gross Unrealized Losses (4,066) (9,588)
Securities held to maturity, fair value 2,278,755 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 192 153
Securities held to maturity, fair value 692 653
U.S. Government-Sponsored Entities [Member]    
Investment Securities Held To Maturity [Line Items]    
Securities Held To Maturity, Amortized Cost 272,715 137,385
Securities Held To Maturity, Gross Unrealized Gains 1,076 809
Securities Held To Maturity, Gross Unrealized Losses (196) (395)
Securities held to maturity, fair value 273,595 137,799
Agency Mortgage-Backed Securities [Member]    
Investment Securities Held To Maturity [Line Items]    
Securities Held To Maturity, Amortized Cost 830,590 709,970
Securities Held To Maturity, Gross Unrealized Gains 20,337 9,858
Securities Held To Maturity, Gross Unrealized Losses   (1,176)
Securities held to maturity, fair value 850,927 718,652
Agency Collateralized Mortgage Obligations [Member]    
Investment Securities Held To Maturity [Line Items]    
Securities Held To Maturity, Amortized Cost 787,120 499,694
Securities Held To Maturity, Gross Unrealized Gains 3,927 803
Securities Held To Maturity, Gross Unrealized Losses (3,054) (7,657)
Securities held to maturity, fair value 787,993 492,840
Non-Agency Collateralized Mortgage Obligations [Member]    
Investment Securities Held To Maturity [Line Items]    
Securities Held To Maturity, Amortized Cost 1,877 2,681
Securities Held To Maturity, Gross Unrealized Gains 8 14
Securities Held To Maturity, Gross Unrealized Losses (5)  
Securities held to maturity, fair value 1,880 2,695
Commercial Mortgage-Backed Securities [Member]    
Investment Securities Held To Maturity [Line Items]    
Securities Held To Maturity, Amortized Cost 50,022 51,258
Securities Held To Maturity, Gross Unrealized Gains 1,478 115
Securities Held To Maturity, Gross Unrealized Losses (120) (259)
Securities held to maturity, fair value 51,380 51,114
States of the U.S. and Political Subdivisions [Member]    
Investment Securities Held To Maturity [Line Items]    
Securities Held To Maturity, Amortized Cost 306,421 235,573
Securities Held To Maturity, Gross Unrealized Gains 6,558 4,191
Securities Held To Maturity, Gross Unrealized Losses (691) (101)
Securities held to maturity, fair value $ 312,288 $ 239,663