Securities - Amortized Cost and Estimated Fair Value of Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Securities Financing Transaction [Line Items]
Available-for-sale, Estimated Fair Value	$ 2,497	$ 29,902
Held-to-maturity, Amortized Cost	480,136	405,361
Held-to-maturity, Estimated Fair Value	478,727	397,763
Total, Amortized Cost	482,625	435,267
Total, Gross Unrealized Gains	6,192	3,626
Total, Gross Unrealized Losses	(7,593)	(11,228)
Total, Estimated Fair Value	481,224	427,665
Held-to-Maturity Securities [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	480,136	405,361
Held-to-maturity, Gross Unrealized Gains	6,184	3,603
Held-to-maturity, Gross Unrealized Losses	(7,593)	(11,201)
Held-to-maturity, Estimated Fair Value	478,727	397,763
Held-to-Maturity Securities [Member] | Investment Securities [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	146,016	124,489
Held-to-maturity, Gross Unrealized Gains	763	105
Held-to-maturity, Gross Unrealized Losses	(7,135)	(8,589)
Held-to-maturity, Estimated Fair Value	139,644	116,005
Held-to-Maturity Securities [Member] | Other investments [Member] | Investment Securities [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	8,731
Held-to-maturity, Gross Unrealized Gains	42
Held-to-maturity, Gross Unrealized Losses	(5)
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	24,958	55,178
Held-to-maturity, Gross Unrealized Gains	344	87
Held-to-maturity, Gross Unrealized Losses		(59)
Held-to-maturity, Estimated Fair Value	25,302	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	36,198	13,311
Held-to-maturity, Gross Unrealized Gains	123	18
Held-to-maturity, Gross Unrealized Losses	(23)	(3)
Held-to-maturity, Estimated Fair Value	36,298	13,326
Corporate Debt Securities [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	55,000
Held-to-maturity, Estimated Fair Value	47,895
Corporate Debt Securities [Member] | Held-to-Maturity Securities [Member] | Investment Securities [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	76,129	56,000
Held-to-maturity, Gross Unrealized Gains	254
Held-to-maturity, Gross Unrealized Losses	(7,107)	(8,527)
Held-to-maturity, Estimated Fair Value	69,276	47,473
Mortgage-Backed Securities [Member] | Held-to-Maturity Securities [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	334,120	280,872
Held-to-maturity, Gross Unrealized Gains	5,421	3,498
Held-to-maturity, Gross Unrealized Losses	(458)	(2,612)
Held-to-maturity, Estimated Fair Value	339,083	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	9,104
Held-to-maturity, Gross Unrealized Gains	40
Held-to-maturity, Estimated Fair Value	9,144
Mortgage-Backed Securities [Member] | Held-to-Maturity Securities [Member] | FHLMC [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	136,934	120,116
Held-to-maturity, Gross Unrealized Gains	1,370	364
Held-to-maturity, Gross Unrealized Losses	(272)	(1,489)
Held-to-maturity, Estimated Fair Value	138,032	118,991
Mortgage-Backed Securities [Member] | Held-to-Maturity Securities [Member] | FNMA [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	177,693	160,254
Held-to-maturity, Gross Unrealized Gains	3,865	3,039
Held-to-maturity, Gross Unrealized Losses	(179)	(1,123)
Held-to-maturity, Estimated Fair Value	181,379	162,170
Mortgage-Backed Securities [Member] | Held-to-Maturity Securities [Member] | GNMA [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	10,389	502
Held-to-maturity, Gross Unrealized Gains	146	95
Held-to-maturity, Gross Unrealized Losses	(7)
Held-to-maturity, Estimated Fair Value	10,528	597
Investment Securities [Member] | U.S. Agency Obligations [Member]
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	2,489	29,906
Available-for-sale, Gross Unrealized Gains	8	23
Available-for-sale, Gross Unrealized Losses		(27)
Available-for-sale, Estimated Fair Value	$ 2,497	$ 29,902