Securities - Estimated Fair Value and Unrealized Loss for Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 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	$ 441,025	$ 372,200
Total securities, Less than 12 months, Unrealized Losses	(5,303)	(5,736)
Total securities, 12 months or longer, Estimated Fair Value	83,190	84,942
Total securities, 12 months or longer, Unrealized Losses	(6,941)	(7,012)
Total securities, Estimated Fair Value	524,215	457,142
Total securities, Unrealized Losses	(12,244)	(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	428,774	359,976
Held-to-maturity, Less than 12 months, Unrealized Losses	(5,014)	(5,418)
Held-to-maturity, 12 months or longer, Estimated Fair Value	83,190	84,942
Held-to-maturity, 12 months or longer, Unrealized Losses	(6,941)	(7,012)
Held-to-maturity, Total, Estimated Fair Value	511,964	444,918
Held-to-maturity, Total, Unrealized Losses	(11,955)	(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	60,587	53,996
Held-to-maturity, Less than 12 months, Unrealized Losses	(878)	(1,204)
Held-to-maturity, 12 months or longer, Estimated Fair Value	49,118	49,119
Held-to-maturity, 12 months or longer, Unrealized Losses	(5,882)	(5,881)
Held-to-maturity, Total, Estimated Fair Value	109,705	103,115
Held-to-maturity, Total, Unrealized Losses	(6,760)	(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	8,588	8,551
Held-to-maturity, Less than 12 months, Unrealized Losses	(232)	(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	8,588	8,551
Held-to-maturity, Total, Unrealized Losses	(232)	(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	12,251	12,224
Available-for-sale, Less than 12 months, Unrealized Losses	(289)	(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	12,251	12,224
Available-for-sale, Total, Unrealized Losses	(289)	(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	43,496
Held-to-maturity, Less than 12 months, Unrealized Losses	(564)
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	43,496
Held-to-maturity, Total, Unrealized Losses	(564)	(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	8,503	12,450
Held-to-maturity, Less than 12 months, Unrealized Losses	(82)	(120)
Held-to-maturity, 12 months or longer, Estimated Fair Value	49,118	49,119
Held-to-maturity, 12 months or longer, Unrealized Losses	(5,882)	(5,881)
Held-to-maturity, Total, Estimated Fair Value	57,621	61,569
Held-to-maturity, Total, Unrealized Losses	(5,964)	(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	368,187	305,980
Held-to-maturity, Less than 12 months, Unrealized Losses	(4,136)	(4,214)
Held-to-maturity, 12 months or longer, Estimated Fair Value	34,072	35,823
Held-to-maturity, 12 months or longer, Unrealized Losses	(1,059)	(1,131)
Held-to-maturity, Total, Estimated Fair Value	402,259	341,803
Held-to-maturity, Total, Unrealized Losses	(5,195)	(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	131,346	102,461
Held-to-maturity, Less than 12 months, Unrealized Losses	(1,545)	(1,665)
Held-to-maturity, 12 months or longer, Estimated Fair Value	25,548	26,898
Held-to-maturity, 12 months or longer, Unrealized Losses	(756)	(792)
Held-to-maturity, Total, Estimated Fair Value	156,894	129,359
Held-to-maturity, Total, Unrealized Losses	(2,301)	(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	155,073	124,403
Held-to-maturity, Less than 12 months, Unrealized Losses	(1,980)	(2,185)
Held-to-maturity, 12 months or longer, Estimated Fair Value	8,524	8,925
Held-to-maturity, 12 months or longer, Unrealized Losses	(303)	(339)
Held-to-maturity, Total, Estimated Fair Value	163,597	133,328
Held-to-maturity, Total, Unrealized Losses	(2,283)	(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	81,768	79,116
Held-to-maturity, Less than 12 months, Unrealized Losses	(611)	(364)
Held-to-maturity, Total, Estimated Fair Value	81,768	79,116
Held-to-maturity, Total, Unrealized Losses	$ (611)	$ (364)