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Securities - Schedule of Amortized Cost and Fair Value of Securities Held to Maturity (Detail) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Investment Securities Held To Maturity [Line Items]      
Securities Held To Maturity, Amortized Cost $ 1,637,061 $ 1,453,355 $ 1,199,169
Securities Held To Maturity, Gross Unrealized Gains 15,943 23,145 15,484
Securities Held To Maturity, Gross Unrealized Losses (9,588) (8,242) (25,090)
Securities held to maturity, fair value 1,643,416 1,468,258 1,189,563
U.S. Treasury [Member]      
Investment Securities Held To Maturity [Line Items]      
Securities Held To Maturity, Amortized Cost 500 502 503
Securities Held To Maturity, Gross Unrealized Gains 153 168 99
Securities held to maturity, fair value 653 670 602
U.S. Government-Sponsored Entities [Member]      
Investment Securities Held To Maturity [Line Items]      
Securities Held To Maturity, Amortized Cost 137,385 101,602 43,322
Securities Held To Maturity, Gross Unrealized Gains 809 885 180
Securities Held To Maturity, Gross Unrealized Losses (395) (524) (1,151)
Securities held to maturity, fair value 137,799 101,963 42,351
Agency Mortgage-Backed Securities [Member]      
Investment Securities Held To Maturity [Line Items]      
Securities Held To Maturity, Amortized Cost 709,970 677,169 628,681
Securities Held To Maturity, Gross Unrealized Gains 9,858 16,712 12,281
Securities Held To Maturity, Gross Unrealized Losses (1,176) (346) (6,032)
Securities held to maturity, fair value 718,652 693,535 634,930
Agency Collateralized Mortgage Obligations [Member]      
Investment Securities Held To Maturity [Line Items]      
Securities Held To Maturity, Amortized Cost 499,694 501,965 385,408
Securities Held To Maturity, Gross Unrealized Gains 803 1,858 764
Securities Held To Maturity, Gross Unrealized Losses (7,657) (7,329) (15,844)
Securities held to maturity, fair value 492,840 496,494 370,328
Non-Agency Collateralized Mortgage Obligations [Member]      
Investment Securities Held To Maturity [Line Items]      
Securities Held To Maturity, Amortized Cost 2,681 4,285 6,852
Securities Held To Maturity, Gross Unrealized Gains 14 28 44
Securities Held To Maturity, Gross Unrealized Losses     (4)
Securities held to maturity, fair value 2,695 4,313 6,892
Commercial Mortgage-Backed Securities [Member]      
Investment Securities Held To Maturity [Line Items]      
Securities Held To Maturity, Amortized Cost 51,258 17,560 2,241
Securities Held To Maturity, Gross Unrealized Gains 115 179 124
Securities Held To Maturity, Gross Unrealized Losses (259)   (37)
Securities held to maturity, fair value 51,114 17,739 2,328
States of the U.S. and Political Subdivisions [Member]      
Investment Securities Held To Maturity [Line Items]      
Securities Held To Maturity, Amortized Cost 235,573 150,272 132,162
Securities Held To Maturity, Gross Unrealized Gains 4,191 3,315 1,992
Securities Held To Maturity, Gross Unrealized Losses (101) (43) (2,022)
Securities held to maturity, fair value $ 239,663 $ 153,544 $ 132,132