Securities - Estimated Fair Value and Unrealized Loss for Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Jun. 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	$ 386,317	$ 372,200
Total securities, Less than 12 months, Unrealized Losses	(3,862)	(5,736)
Total securities, 12 months or longer, Estimated Fair Value	87,182	84,942
Total securities, 12 months or longer, Unrealized Losses	(4,739)	(7,012)
Total securities, Estimated Fair Value	473,499	457,142
Total securities, Unrealized Losses	(8,601)	(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	349,045	359,976
Held-to-maturity, Less than 12 months, Unrealized Losses	(3,548)	(5,418)
Held-to-maturity, 12 months or longer, Estimated Fair Value	87,182	84,942
Held-to-maturity, 12 months or longer, Unrealized Losses	(4,739)	(7,012)
Held-to-maturity, Total, Estimated Fair Value	436,227	444,918
Held-to-maturity, Total, Unrealized Losses	(8,287)	(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	76,690	53,996
Held-to-maturity, Less than 12 months, Unrealized Losses	(622)	(1,204)
Held-to-maturity, 12 months or longer, Estimated Fair Value	51,469	49,119
Held-to-maturity, 12 months or longer, Unrealized Losses	(3,783)	(5,881)
Held-to-maturity, Total, Estimated Fair Value	128,159	103,115
Held-to-maturity, Total, Unrealized Losses	(4,405)	(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	8,670	8,551
Held-to-maturity, Less than 12 months, Unrealized Losses	(191)	(228)
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	8,670	8,551
Held-to-maturity, Total, Unrealized Losses	(191)	(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	37,272	12,224
Available-for-sale, Less than 12 months, Unrealized Losses	(314)	(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	37,272	12,224
Available-for-sale, Total, Unrealized Losses	(314)	(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	61,001
Held-to-maturity, Less than 12 months, Unrealized Losses	(400)
Held-to-maturity, 12 months or longer, Estimated Fair Value	250
Held-to-maturity, 12 months or longer, Unrealized Losses	(2)
Held-to-maturity, Total, Estimated Fair Value	61,251
Held-to-maturity, Total, Unrealized Losses	(402)	(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	7,019	12,450
Held-to-maturity, Less than 12 months, Unrealized Losses	(31)	(120)
Held-to-maturity, 12 months or longer, Estimated Fair Value	51,219	49,119
Held-to-maturity, 12 months or longer, Unrealized Losses	(3,781)	(5,881)
Held-to-maturity, Total, Estimated Fair Value	58,238	61,569
Held-to-maturity, Total, Unrealized Losses	(3,812)	(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	272,355	305,980
Held-to-maturity, Less than 12 months, Unrealized Losses	(2,926)	(4,214)
Held-to-maturity, 12 months or longer, Estimated Fair Value	35,713	35,823
Held-to-maturity, 12 months or longer, Unrealized Losses	(956)	(1,131)
Held-to-maturity, Total, Estimated Fair Value	308,068	341,803
Held-to-maturity, Total, Unrealized Losses	(3,882)	(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	84,056	102,461
Held-to-maturity, Less than 12 months, Unrealized Losses	(1,079)	(1,665)
Held-to-maturity, 12 months or longer, Estimated Fair Value	25,232	26,898
Held-to-maturity, 12 months or longer, Unrealized Losses	(673)	(792)
Held-to-maturity, Total, Estimated Fair Value	109,288	129,359
Held-to-maturity, Total, Unrealized Losses	(1,752)	(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	124,334	124,403
Held-to-maturity, Less than 12 months, Unrealized Losses	(1,454)	(2,185)
Held-to-maturity, 12 months or longer, Estimated Fair Value	10,481	8,925
Held-to-maturity, 12 months or longer, Unrealized Losses	(283)	(339)
Held-to-maturity, Total, Estimated Fair Value	134,815	133,328
Held-to-maturity, Total, Unrealized Losses	(1,737)	(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	63,965	79,116
Held-to-maturity, Less than 12 months, Unrealized Losses	(393)	(364)
Held-to-maturity, Total, Estimated Fair Value	63,965	79,116
Held-to-maturity, Total, Unrealized Losses	$ (393)	$ (364)