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Securities - Schedule of Amortized Cost and Fair Value of Securities Held to Maturity (Detail) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Investment Securities Held To Maturity [Line Items]      
Securities Held To Maturity, Amortized Cost $ 2,337,342 $ 1,637,061 $ 1,453,355
Securities Held To Maturity, Gross Unrealized Gains 8,226 15,943 23,145
Securities Held To Maturity, Gross Unrealized Losses (50,791) (9,588) (8,242)
Securities held to maturity, fair value 2,294,777 1,643,416 1,468,258
U.S. Treasury [Member]      
Investment Securities Held To Maturity [Line Items]      
Securities Held To Maturity, Amortized Cost 500 500 502
Securities Held To Maturity, Gross Unrealized Gains 137 153 168
Securities held to maturity, fair value 637 653 670
U.S. Government-Sponsored Entities [Member]      
Investment Securities Held To Maturity [Line Items]      
Securities Held To Maturity, Amortized Cost 272,645 137,385 101,602
Securities Held To Maturity, Gross Unrealized Gains 348 809 885
Securities Held To Maturity, Gross Unrealized Losses (4,475) (395) (524)
Securities held to maturity, fair value 268,518 137,799 101,963
Agency Mortgage-Backed Securities [Member]      
Investment Securities Held To Maturity [Line Items]      
Securities Held To Maturity, Amortized Cost 852,215 709,970 677,169
Securities Held To Maturity, Gross Unrealized Gains 5,654 9,858 16,712
Securities Held To Maturity, Gross Unrealized Losses (8,645) (1,176) (346)
Securities held to maturity, fair value 849,224 718,652 693,535
Agency Collateralized Mortgage Obligations [Member]      
Investment Securities Held To Maturity [Line Items]      
Securities Held To Maturity, Amortized Cost 743,148 499,694 501,965
Securities Held To Maturity, Gross Unrealized Gains 447 803 1,858
Securities Held To Maturity, Gross Unrealized Losses (17,801) (7,657) (7,329)
Securities held to maturity, fair value 725,794 492,840 496,494
Non-Agency Collateralized Mortgage Obligations [Member]      
Investment Securities Held To Maturity [Line Items]      
Securities Held To Maturity, Amortized Cost 1,689 2,681 4,285
Securities Held To Maturity, Gross Unrealized Gains 3 14 28
Securities Held To Maturity, Gross Unrealized Losses (6)    
Securities held to maturity, fair value 1,686 2,695 4,313
Commercial Mortgage-Backed Securities [Member]      
Investment Securities Held To Maturity [Line Items]      
Securities Held To Maturity, Amortized Cost 49,797 51,258 17,560
Securities Held To Maturity, Gross Unrealized Gains 181 115 179
Securities Held To Maturity, Gross Unrealized Losses (226) (259)  
Securities held to maturity, fair value 49,752 51,114 17,739
States of the U.S. and Political Subdivisions [Member]      
Investment Securities Held To Maturity [Line Items]      
Securities Held To Maturity, Amortized Cost 417,348 235,573 150,272
Securities Held To Maturity, Gross Unrealized Gains 1,456 4,191 3,315
Securities Held To Maturity, Gross Unrealized Losses (19,638) (101) (43)
Securities held to maturity, fair value $ 399,166 $ 239,663 $ 153,544