Securities - Schedule of Unrealized Losses and Fair Value by Major Security Type (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value
Less Than 12 Months	$ 84,666	$ 293,410
12 Months or Longer	98,930	579,927
Total	183,596	873,337
Unrealized Losses
Less Than 12 Months	(346)	(1,910)
12 Months or Longer	(789)	(19,829)
Total	(1,135)	(21,739)
Fair Value
Less Than 12 Months		10,601
12 Months or Longer		71,742
Total		82,343
Unrecognized Losses
Less Than 12 Months		(79)
12 Months or Longer		(3,083)
Total		(3,162)
U.S. Treasury securities
Fair Value
Less Than 12 Months		163,722
12 Months or Longer		0
Total		163,722
Unrealized Losses
Less Than 12 Months		(60)
12 Months or Longer		0
Total		(60)
U.S. government sponsored entities and agencies
Fair Value
Less Than 12 Months		1,355
12 Months or Longer		19,937
Total		21,292
Unrealized Losses
Less Than 12 Months		(12)
12 Months or Longer		(100)
Total		(112)
Mortgage-backed securities: residential
Fair Value
Less Than 12 Months	49,390	83,203
12 Months or Longer	91,644	490,752
Total	141,034	573,955
Unrealized Losses
Less Than 12 Months	(172)	(755)
12 Months or Longer	(684)	(15,339)
Total	(856)	(16,094)
Fair Value
Less Than 12 Months		2,239
12 Months or Longer		68,067
Total		70,306
Unrecognized Losses
Less Than 12 Months		(40)
12 Months or Longer		(3,032)
Total		(3,072)
Mortgage-backed securities: commercial
Fair Value
Less Than 12 Months	4,436	24,845
12 Months or Longer	7,286	0
Total	11,722	24,845
Unrealized Losses
Less Than 12 Months	(29)	(900)
12 Months or Longer	(105)	0
Total	(134)	(900)
Corporate Notes
Fair Value
Less Than 12 Months	997	9,839
12 Months or Longer	0	0
Total	997	9,839
Unrealized Losses
Less Than 12 Months	(3)	(77)
12 Months or Longer	0	0
Total	(3)	(77)
State and political subdivisions
Fair Value
Less Than 12 Months	29,843	10,446
12 Months or Longer	0	69,238
Total	29,843	79,684
Unrealized Losses
Less Than 12 Months	(142)	(106)
12 Months or Longer	0	(4,390)
Total	$ (142)	(4,496)
Fair Value
Less Than 12 Months		8,362
12 Months or Longer		3,675
Total		12,037
Unrecognized Losses
Less Than 12 Months		(39)
12 Months or Longer		(51)
Total		$ (90)