Securities - Amortized Cost and Fair Value of Held to Maturity Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items]
Securities Held to Maturity, Amortized Cost	$ 2,294,318	$ 2,166,289
Securities Held to Maturity, Gross Unrealized Gains	27,699	32,903
Securities Held to Maturity, Gross Unrealized Losses	11,914	12,852
Securities Held to Maturity, Fair Value	2,310,103	2,186,340
U.S. Treasury and Government Agency Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities Held to Maturity, Amortized Cost	200,000
Securities Held to Maturity, Gross Unrealized Gains	55
Securities Held to Maturity, Gross Unrealized Losses	770
Securities Held to Maturity, Fair Value	199,285
Municipal Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities Held to Maturity, Amortized Cost	186,528	180,615
Securities Held to Maturity, Gross Unrealized Gains	3,074	3,416
Securities Held to Maturity, Gross Unrealized Losses	1,834	1,144
Securities Held to Maturity, Fair Value	187,768	182,887
Mortgage-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities Held to Maturity, Amortized Cost	898,638	899,923
Securities Held to Maturity, Gross Unrealized Gains	19,378	23,897
Securities Held to Maturity, Gross Unrealized Losses	150	162
Securities Held to Maturity, Fair Value	917,866	923,658
Collateralized Mortgage Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities Held to Maturity, Amortized Cost	1,009,152	1,085,751
Securities Held to Maturity, Gross Unrealized Gains	5,192	5,590
Securities Held to Maturity, Gross Unrealized Losses	9,160	11,546
Securities Held to Maturity, Fair Value	$ 1,005,184	$ 1,079,795

Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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-Name Accounting Standards Codification

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