Securities - Amortized Cost and Estimated Fair Value of Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Securities Financing Transaction [Line Items]
Available-for-sale, Estimated Fair Value	$ 12,509	$ 29,902
Held-to-maturity, Amortized Cost	523,590	405,361
Held-to-maturity, Estimated Fair Value	520,971	397,763
Total, Amortized Cost	536,084	435,267
Total, Gross Unrealized Gains	7,268	3,626
Total, Gross Unrealized Losses	(9,872)	(11,228)
Total, Estimated Fair Value	533,480	427,665
Held-to-Maturity Securities [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	523,590	405,361
Held-to-maturity, Gross Unrealized Gains	7,250	3,603
Held-to-maturity, Gross Unrealized Losses	(9,869)	(11,201)
Held-to-maturity, Estimated Fair Value	520,971	397,763
Held-to-Maturity Securities [Member] | Investment Securities [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	168,023	124,489
Held-to-maturity, Gross Unrealized Gains	977	105
Held-to-maturity, Gross Unrealized Losses	(9,597)	(8,589)
Held-to-maturity, Estimated Fair Value	159,403	116,005
Held-to-Maturity Securities [Member] | Other investments [Member] | Investment Securities [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	8,702
Held-to-maturity, Gross Unrealized Gains	66
Held-to-maturity, Estimated Fair Value	8,768
U.S. Agency Obligations [Member] | Held-to-Maturity Securities [Member] | Investment Securities [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	44,981	55,178
Held-to-maturity, Gross Unrealized Gains	460	87
Held-to-maturity, Gross Unrealized Losses		(59)
Held-to-maturity, Estimated Fair Value	45,441	55,206
State and Municipal Obligations [Member] | Held-to-Maturity Securities [Member] | Investment Securities [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	38,197	13,311
Held-to-maturity, Gross Unrealized Gains	252	18
Held-to-maturity, Gross Unrealized Losses	(2)	(3)
Held-to-maturity, Estimated Fair Value	38,447	13,326
Corporate Debt Securities [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	55,000
Held-to-maturity, Estimated Fair Value	45,406
Corporate Debt Securities [Member] | Held-to-Maturity Securities [Member] | Investment Securities [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	76,143	56,000
Held-to-maturity, Gross Unrealized Gains	199
Held-to-maturity, Gross Unrealized Losses	(9,595)	(8,527)
Held-to-maturity, Estimated Fair Value	66,747	47,473
Mortgage-Backed Securities [Member] | Held-to-Maturity Securities [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	355,567	280,872
Held-to-maturity, Gross Unrealized Gains	6,273	3,498
Held-to-maturity, Gross Unrealized Losses	(272)	(2,612)
Held-to-maturity, Estimated Fair Value	361,568	281,758
Mortgage-Backed Securities [Member] | Held-to-Maturity Securities [Member] | Other mortgage-backed securities [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	9,562
Held-to-maturity, Gross Unrealized Gains	21
Held-to-maturity, Estimated Fair Value	9,583
Mortgage-Backed Securities [Member] | Held-to-Maturity Securities [Member] | FHLMC [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	145,664	120,116
Held-to-maturity, Gross Unrealized Gains	1,641	364
Held-to-maturity, Gross Unrealized Losses	(168)	(1,489)
Held-to-maturity, Estimated Fair Value	147,137	118,991
Mortgage-Backed Securities [Member] | Held-to-Maturity Securities [Member] | FNMA [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	189,368	160,254
Held-to-maturity, Gross Unrealized Gains	4,428	3,039
Held-to-maturity, Gross Unrealized Losses	(104)	(1,123)
Held-to-maturity, Estimated Fair Value	193,692	162,170
Mortgage-Backed Securities [Member] | Held-to-Maturity Securities [Member] | GNMA [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	10,973	502
Held-to-maturity, Gross Unrealized Gains	183	95
Held-to-maturity, Estimated Fair Value	11,156	597
Investment Securities [Member] | U.S. Agency Obligations [Member]
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	12,494	29,906
Available-for-sale, Gross Unrealized Gains	18	23
Available-for-sale, Gross Unrealized Losses	(3)	(27)
Available-for-sale, Estimated Fair Value	$ 12,509	$ 29,902