Securities - Estimated Fair Value and Unrealized Loss for Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Total securities, Less than 12 months, Estimated Fair Value	$ 679,517	$ 450,421
Total securities, Less than 12 months, Unrealized Losses	(10,837)	(3,806)
Total securities, 12 months or longer, Estimated Fair Value	260,056	261,254
Total securities, 12 months or longer, Unrealized Losses	(11,516)	(9,358)
Total securities, Estimated Fair Value	939,573	711,675
Total securities, Unrealized Losses	(22,353)	(13,164)
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	615,501	381,046
Held-to-maturity, Less than 12 months, Unrealized Losses	(9,981)	(3,310)
Held-to-maturity, 12 months or longer, Estimated Fair Value	237,958	249,048
Held-to-maturity, 12 months or longer, Unrealized Losses	(11,108)	(9,057)
Held-to-maturity, Total, Estimated Fair Value	853,459	630,094
Held-to-maturity, Total, Unrealized Losses	(21,089)	(12,367)
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	142,331	123,821
Held-to-maturity, Less than 12 months, Unrealized Losses	(2,332)	(1,248)
Held-to-maturity, 12 months or longer, Estimated Fair Value	68,799	70,469
Held-to-maturity, 12 months or longer, Unrealized Losses	(4,335)	(4,254)
Held-to-maturity, Total, Estimated Fair Value	211,130	194,290
Held-to-maturity, Total, Unrealized Losses	(6,667)	(5,502)
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	7,414	4,035
Held-to-maturity, Less than 12 months, Unrealized Losses	(95)	(30)
Held-to-maturity, 12 months or longer, Estimated Fair Value	55,252	56,106
Held-to-maturity, 12 months or longer, Unrealized Losses	(3,782)	(3,932)
Held-to-maturity, Total, Estimated Fair Value	62,666	60,141
Held-to-maturity, Total, Unrealized Losses	(3,877)	(3,962)
U.S. Agency Obligations | Investment Securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Available-for-sale, 12 months or longer, Estimated Market Value		0
Available-for-sale, 12 months or longer, Unrealized Losses		0
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		14,903
Held-to-maturity, Less than 12 months, Unrealized Losses		(65)
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		14,903
Held-to-maturity, Total, Unrealized Losses		(65)
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	473,170	257,225
Held-to-maturity, Less than 12 months, Unrealized Losses	(7,649)	(2,062)
Held-to-maturity, 12 months or longer, Estimated Fair Value	169,159	178,579
Held-to-maturity, 12 months or longer, Unrealized Losses	(6,773)	(4,803)
Held-to-maturity, Total, Estimated Fair Value	642,329	435,804
Held-to-maturity, Total, Unrealized Losses	(14,422)	(6,865)
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	202,453	98,138
Held-to-maturity, Less than 12 months, Unrealized Losses	(2,921)	(781)
Held-to-maturity, 12 months or longer, Estimated Fair Value	64,716	68,238
Held-to-maturity, 12 months or longer, Unrealized Losses	(2,868)	(2,156)
Held-to-maturity, Total, Estimated Fair Value	267,169	166,376
Held-to-maturity, Total, Unrealized Losses	(5,789)	(2,937)
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	199,147	132,982
Held-to-maturity, Less than 12 months, Unrealized Losses	(3,877)	(1,058)
Held-to-maturity, 12 months or longer, Estimated Fair Value	61,253	65,060
Held-to-maturity, 12 months or longer, Unrealized Losses	(2,802)	(1,942)
Held-to-maturity, Total, Estimated Fair Value	260,400	198,042
Held-to-maturity, Total, Unrealized Losses	(6,679)	(3,000)
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	66,529	26,105
Held-to-maturity, Less than 12 months, Unrealized Losses	(849)	(223)
Held-to-maturity, 12 months or longer, Estimated Fair Value	43,190	45,281
Held-to-maturity, 12 months or longer, Unrealized Losses	(1,103)	(705)
Held-to-maturity, Total, Estimated Fair Value	109,719	71,386
Held-to-maturity, Total, Unrealized Losses	(1,952)	(928)
Mortgage-Backed Securities | Held-to-Maturity Securities | SBA
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value	5,041
Held-to-maturity, Less than 12 months, Unrealized Losses	(2)
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	5,041
Held-to-maturity, Total, Unrealized Losses	(2)	0
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	14,816
Held-to-maturity, Less than 12 months, Unrealized Losses	(154)
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	14,816
Held-to-maturity, Total, Unrealized Losses	(154)
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	120,101	104,883
Held-to-maturity, Less than 12 months, Unrealized Losses	(2,083)	(1,153)
Held-to-maturity, 12 months or longer, Estimated Fair Value	13,547	14,363
Held-to-maturity, 12 months or longer, Unrealized Losses	(553)	(322)
Held-to-maturity, Total, Estimated Fair Value	133,648	119,246
Held-to-maturity, Total, Unrealized Losses	(2,636)	(1,475)
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	64,016
Held-to-maturity, Less than 12 months, Unrealized Losses	(856)
Held-to-maturity, 12 months or longer, Estimated Fair Value	22,098
Held-to-maturity, 12 months or longer, Unrealized Losses	(408)
Held-to-maturity, Total, Estimated Fair Value	86,114
Held-to-maturity, Total, Unrealized Losses	(1,264)
Investment Securities | U.S. Agency Obligations
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Available-for-sale, Less than 12 months, Estimated Market Value	62,502	69,375
Available-for-sale, Less than 12 months, Unrealized Losses	(853)	(496)
Available-for-sale, 12 months or longer, Estimated Market Value	22,098	12,206
Available-for-sale, 12 months or longer, Unrealized Losses	(408)	(301)
Available-for-sale, Total, Estimated Market Value	84,600	81,581
Available-for-sale, Total, Unrealized Losses	(1,261)	$ (797)
Investment Securities | Mortgage-Backed Securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value	1,514
Held-to-maturity, Less than 12 months, Unrealized Losses	(3)
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	1,514
Held-to-maturity, Total, Unrealized Losses	$ (3)