Securities - Estimated Fair Value and Unrealized Loss for Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Total securities, Less than 12 months, Estimated Fair Value	$ 364,793	$ 372,200
Total securities, Less than 12 months, Unrealized Losses	(3,187)	(5,736)
Total securities, 12 months or longer, Estimated Fair Value	123,185	84,942
Total securities, 12 months or longer, Unrealized Losses	(5,309)	(7,012)
Total securities, Estimated Fair Value	487,978	457,142
Total securities, Unrealized Losses	(8,496)	(12,748)
Held-to-Maturity Securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value	312,517	359,976
Held-to-maturity, Less than 12 months, Unrealized Losses	(2,865)	(5,418)
Held-to-maturity, 12 months or longer, Estimated Fair Value	123,185	84,942
Held-to-maturity, 12 months or longer, Unrealized Losses	(5,309)	(7,012)
Held-to-maturity, Total, Estimated Fair Value	435,702	444,918
Held-to-maturity, Total, Unrealized Losses	(8,174)	(12,430)
Held-to-Maturity Securities | Investment Securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value	83,007	53,996
Held-to-maturity, Less than 12 months, Unrealized Losses	(542)	(1,204)
Held-to-maturity, 12 months or longer, Estimated Fair Value	71,882	49,119
Held-to-maturity, 12 months or longer, Unrealized Losses	(3,979)	(5,881)
Held-to-maturity, Total, Estimated Fair Value	154,889	103,115
Held-to-maturity, Total, Unrealized Losses	(4,521)	(7,085)
Held-to-Maturity Securities | Investment Securities | Other investments
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value	0	8,551
Held-to-maturity, Less than 12 months, Unrealized Losses	0	(228)
Held-to-maturity, 12 months or longer, Estimated Fair Value	8,714	0
Held-to-maturity, 12 months or longer, Unrealized Losses	(189)	0
Held-to-maturity, Total, Estimated Fair Value	8,714	8,551
Held-to-maturity, Total, Unrealized Losses	(189)	(228)
U.S. Agency Obligations | Investment Securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Available-for-sale, Less than 12 months, Estimated Market Value	52,276	12,224
Available-for-sale, Less than 12 months, Unrealized Losses	(322)	(318)
Available-for-sale, 12 months or longer, Estimated Market Value		0
Available-for-sale, 12 months or longer, Unrealized Losses		0
Available-for-sale, Total, Estimated Market Value	52,276	12,224
Available-for-sale, Total, Unrealized Losses	(322)	(318)
U.S. Agency Obligations | Held-to-Maturity Securities | Investment Securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value		32,995
Held-to-maturity, Less than 12 months, Unrealized Losses		(856)
Held-to-maturity, 12 months or longer, Estimated Fair Value		0
Held-to-maturity, 12 months or longer, Unrealized Losses		0
Held-to-maturity, Total, Estimated Fair Value		32,995
Held-to-maturity, Total, Unrealized Losses		(856)
State and Municipal Obligations | Held-to-Maturity Securities | Investment Securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value	77,979
Held-to-maturity, Less than 12 months, Unrealized Losses	(531)
Held-to-maturity, 12 months or longer, Estimated Fair Value	9,908
Held-to-maturity, 12 months or longer, Unrealized Losses	(48)
Held-to-maturity, Total, Estimated Fair Value	87,887
Held-to-maturity, Total, Unrealized Losses	(579)	(856)
Corporate Debt Securities | Held-to-Maturity Securities | Investment Securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value	5,028	12,450
Held-to-maturity, Less than 12 months, Unrealized Losses	(11)	(120)
Held-to-maturity, 12 months or longer, Estimated Fair Value	53,260	49,119
Held-to-maturity, 12 months or longer, Unrealized Losses	(3,742)	(5,881)
Held-to-maturity, Total, Estimated Fair Value	58,288	61,569
Held-to-maturity, Total, Unrealized Losses	(3,753)	(6,001)
Mortgage-Backed Securities | Held-to-Maturity Securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value	229,510	305,980
Held-to-maturity, Less than 12 months, Unrealized Losses	(2,323)	(4,214)
Held-to-maturity, 12 months or longer, Estimated Fair Value	51,303	35,823
Held-to-maturity, 12 months or longer, Unrealized Losses	(1,330)	(1,131)
Held-to-maturity, Total, Estimated Fair Value	280,813	341,803
Held-to-maturity, Total, Unrealized Losses	(3,653)	(5,345)
Mortgage-Backed Securities | Held-to-Maturity Securities | FHLMC
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value	67,059	102,461
Held-to-maturity, Less than 12 months, Unrealized Losses	(892)	(1,665)
Held-to-maturity, 12 months or longer, Estimated Fair Value	30,684	26,898
Held-to-maturity, 12 months or longer, Unrealized Losses	(782)	(792)
Held-to-maturity, Total, Estimated Fair Value	97,743	129,359
Held-to-maturity, Total, Unrealized Losses	(1,674)	(2,457)
Mortgage-Backed Securities | Held-to-Maturity Securities | FNMA
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value	96,886	124,403
Held-to-maturity, Less than 12 months, Unrealized Losses	(954)	(2,185)
Held-to-maturity, 12 months or longer, Estimated Fair Value	19,878	8,925
Held-to-maturity, 12 months or longer, Unrealized Losses	(520)	(339)
Held-to-maturity, Total, Estimated Fair Value	116,764	133,328
Held-to-maturity, Total, Unrealized Losses	(1,474)	(2,524)
Mortgage-Backed Securities | Held-to-Maturity Securities | GNMA
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value	65,565	79,116
Held-to-maturity, Less than 12 months, Unrealized Losses	(477)	(364)
Held-to-maturity, 12 months or longer, Estimated Fair Value	741
Held-to-maturity, 12 months or longer, Unrealized Losses	(28)
Held-to-maturity, Total, Estimated Fair Value	66,306	79,116
Held-to-maturity, Total, Unrealized Losses	$ (505)	$ (364)