SECURITIES - Fair Values and Unrealized Losses of Impaired Securities, by Length of Impairment (Detail) $ in Thousands	Sep. 30, 2017 USD ($) Security	Dec. 31, 2016 USD ($) Security
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year | Security	85	113
Number of available for sale securities, Greater than 1 year | Security	24	12
Number of available for sale securities | Security	109	125
Securities Available For Sale, Less than 12 Months, Fair Value	$ 1,645,698	$ 1,644,951
Securities Available For Sale, Less than 12 Months, Unrealized Losses	(13,907)	(27,928)
Securities Available For Sale, 12 Months or More, Fair Value	218,224	93,510
Securities Available For Sale, 12 Months or More, Unrealized Losses	(6,814)	(4,278)
Securities Available For Sale, Fair Value, Total	1,863,922	1,738,461
Securities Available For Sale, Unrealized Losses, Total	$ (20,721)	$ (32,206)
Number of held to maturity securities | Security	147	186
Securities Held To Maturity, Less than 12 Months, Fair Value	$ 1,083,335	$ 1,513,912
Securities Held To Maturity, Less than 12 Months, Unrealized Losses	(15,914)	(46,484)
Securities Held To Maturity, Greater than 12 Months, Fair Value	411,851	120,957
Securities Held To Maturity, Greater than 12 Months, Unrealized Losses	(16,821)	(4,307)
Securities Held To Maturity, Fair Value, Total	1,495,186	1,634,869
Securities Held To Maturity, Unrealized Losses, Total	$ (32,735)	$ (50,791)
Less than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities | Security	109	173
Greater than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities | Security	38	13
U.S. Government-Sponsored Entities [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year | Security	10	11
Number of available for sale securities, Greater than 1 year | Security	2	0
Number of available for sale securities | Security	12	11
Securities Available For Sale, Less than 12 Months, Fair Value	$ 193,778	$ 211,636
Securities Available For Sale, Less than 12 Months, Unrealized Losses	(1,219)	(3,370)
Securities Available For Sale, 12 Months or More, Fair Value	38,754	0
Securities Available For Sale, 12 Months or More, Unrealized Losses	(1,246)	0
Securities Available For Sale, Fair Value, Total	232,532	211,636
Securities Available For Sale, Unrealized Losses, Total	$ (2,465)	$ (3,370)
Number of held to maturity securities | Security	11	10
Securities Held To Maturity, Less than 12 Months, Fair Value	$ 162,966	$ 185,525
Securities Held To Maturity, Less than 12 Months, Unrealized Losses	(2,035)	(4,475)
Securities Held To Maturity, Greater than 12 Months, Fair Value	38,621	0
Securities Held To Maturity, Greater than 12 Months, Unrealized Losses	(1,378)	0
Securities Held To Maturity, Fair Value, Total	201,587	185,525
Securities Held To Maturity, Unrealized Losses, Total	$ (3,413)	$ (4,475)
U.S. Government-Sponsored Entities [Member] | Less than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities | Security	9	10
U.S. Government-Sponsored Entities [Member] | Greater than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities | Security	2	0
Agency Mortgage-Backed Securities [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year | Security	49	55
Number of available for sale securities, Greater than 1 year | Security	0	0
Number of available for sale securities | Security	49	55
Securities Available For Sale, Less than 12 Months, Fair Value	$ 1,032,211	$ 1,056,731
Securities Available For Sale, Less than 12 Months, Unrealized Losses	(7,828)	(16,994)
Securities Available For Sale, 12 Months or More, Fair Value	0	0
Securities Available For Sale, 12 Months or More, Unrealized Losses	0	0
Securities Available For Sale, Fair Value, Total	1,032,211	1,056,731
Securities Available For Sale, Unrealized Losses, Total	$ (7,828)	$ (16,994)
Number of held to maturity securities | Security	29	36
Securities Held To Maturity, Less than 12 Months, Fair Value	$ 518,680	$ 551,404
Securities Held To Maturity, Less than 12 Months, Unrealized Losses	(4,355)	(8,645)
Securities Held To Maturity, Greater than 12 Months, Fair Value	0	0
Securities Held To Maturity, Greater than 12 Months, Unrealized Losses	0	0
Securities Held To Maturity, Fair Value, Total	518,680	551,404
Securities Held To Maturity, Unrealized Losses, Total	$ (4,355)	$ (8,645)
Agency Mortgage-Backed Securities [Member] | Less than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities | Security	29	36
Agency Mortgage-Backed Securities [Member] | Greater than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities | Security	0	0
Agency Collateralized Mortgage Obligations [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year | Security	21	26
Number of available for sale securities, Greater than 1 year | Security	18	9
Number of available for sale securities | Security	39	35
Securities Available For Sale, Less than 12 Months, Fair Value	$ 411,642	$ 346,662
Securities Available For Sale, Less than 12 Months, Unrealized Losses	(4,846)	(7,261)
Securities Available For Sale, 12 Months or More, Fair Value	173,880	89,040
Securities Available For Sale, 12 Months or More, Unrealized Losses	(5,357)	(3,843)
Securities Available For Sale, Fair Value, Total	585,522	435,702
Securities Available For Sale, Unrealized Losses, Total	$ (10,203)	$ (11,104)
Number of held to maturity securities | Security	38	41
Securities Held To Maturity, Less than 12 Months, Fair Value	$ 197,186	$ 516,237
Securities Held To Maturity, Less than 12 Months, Unrealized Losses	(2,462)	(13,710)
Securities Held To Maturity, Greater than 12 Months, Fair Value	332,491	112,690
Securities Held To Maturity, Greater than 12 Months, Unrealized Losses	(11,241)	(4,091)
Securities Held To Maturity, Fair Value, Total	529,677	628,927
Securities Held To Maturity, Unrealized Losses, Total	$ (13,703)	$ (17,801)
Agency Collateralized Mortgage Obligations [Member] | Less than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities | Security	15	29
Agency Collateralized Mortgage Obligations [Member] | Greater than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities | Security	23	12
Non-Agency Collateralized Mortgage Obligations [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year | Security	0
Number of available for sale securities, Greater than 1 year | Security	0
Number of available for sale securities | Security	0
Securities Available For Sale, Less than 12 Months, Fair Value	$ 0
Securities Available For Sale, Less than 12 Months, Unrealized Losses	0
Securities Available For Sale, 12 Months or More, Fair Value	0
Securities Available For Sale, 12 Months or More, Unrealized Losses	0
Securities Available For Sale, Fair Value, Total	0
Securities Available For Sale, Unrealized Losses, Total	$ 0
Number of held to maturity securities | Security		3
Securities Held To Maturity, Less than 12 Months, Fair Value		$ 1,128
Securities Held To Maturity, Less than 12 Months, Unrealized Losses		(6)
Securities Held To Maturity, Greater than 12 Months, Fair Value		0
Securities Held To Maturity, Greater than 12 Months, Unrealized Losses		0
Securities Held To Maturity, Fair Value, Total		1,128
Securities Held To Maturity, Unrealized Losses, Total		$ (6)
Non-Agency Collateralized Mortgage Obligations [Member] | Less than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities | Security		3
Non-Agency Collateralized Mortgage Obligations [Member] | Greater than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities | Security		0
Commercial Mortgage-Backed Securities [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year | Security	0	1
Number of available for sale securities, Greater than 1 year | Security	0	0
Number of available for sale securities | Security	0	1
Securities Available For Sale, Less than 12 Months, Fair Value	$ 0	$ 1,291
Securities Available For Sale, Less than 12 Months, Unrealized Losses	0	(1)
Securities Available For Sale, 12 Months or More, Fair Value	0	0
Securities Available For Sale, 12 Months or More, Unrealized Losses	0	0
Securities Available For Sale, Fair Value, Total	0	1,291
Securities Available For Sale, Unrealized Losses, Total	$ 0	$ (1)
Number of held to maturity securities | Security	4	2
Securities Held To Maturity, Less than 12 Months, Fair Value	$ 14,730	$ 12,317
Securities Held To Maturity, Less than 12 Months, Unrealized Losses	(106)	(10)
Securities Held To Maturity, Greater than 12 Months, Fair Value	7,562	8,267
Securities Held To Maturity, Greater than 12 Months, Unrealized Losses	(251)	(216)
Securities Held To Maturity, Fair Value, Total	22,292	20,584
Securities Held To Maturity, Unrealized Losses, Total	$ (357)	$ (226)
Commercial Mortgage-Backed Securities [Member] | Less than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities | Security	3	1
Commercial Mortgage-Backed Securities [Member] | Greater than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities | Security	1	1
States of the U.S. and Political Subdivisions [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year | Security	4	20
Number of available for sale securities, Greater than 1 year | Security	1	0
Number of available for sale securities | Security	5	20
Securities Available For Sale, Less than 12 Months, Fair Value	$ 7,882	$ 28,631
Securities Available For Sale, Less than 12 Months, Unrealized Losses	(6)	(302)
Securities Available For Sale, 12 Months or More, Fair Value	886	0
Securities Available For Sale, 12 Months or More, Unrealized Losses	(3)	0
Securities Available For Sale, Fair Value, Total	8,768	28,631
Securities Available For Sale, Unrealized Losses, Total	$ (9)	$ (302)
Number of held to maturity securities | Security	65	94
Securities Held To Maturity, Less than 12 Months, Fair Value	$ 189,773	$ 247,301
Securities Held To Maturity, Less than 12 Months, Unrealized Losses	(6,956)	(19,638)
Securities Held To Maturity, Greater than 12 Months, Fair Value	33,177	0
Securities Held To Maturity, Greater than 12 Months, Unrealized Losses	(3,951)	0
Securities Held To Maturity, Fair Value, Total	222,950	247,301
Securities Held To Maturity, Unrealized Losses, Total	$ (10,907)	$ (19,638)
States of the U.S. and Political Subdivisions [Member] | Less than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities | Security	53	94
States of the U.S. and Political Subdivisions [Member] | Greater than 12 Months [Member]
Schedule Of Securities [Line Items]
Number of held to maturity securities | Security	12	0
Other Debt Securities [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year | Security	0	0
Number of available for sale securities, Greater than 1 year | Security	3	3
Number of available for sale securities | Security	3	3
Securities Available For Sale, Less than 12 Months, Fair Value	$ 0	$ 0
Securities Available For Sale, Less than 12 Months, Unrealized Losses	0	0
Securities Available For Sale, 12 Months or More, Fair Value	4,704	4,470
Securities Available For Sale, 12 Months or More, Unrealized Losses	(208)	(435)
Securities Available For Sale, Fair Value, Total	4,704	4,470
Securities Available For Sale, Unrealized Losses, Total	$ (208)	$ (435)
Equity Securities [Member]
Schedule Of Securities [Line Items]
Number of available for sale securities, Less than 1 year | Security	1
Number of available for sale securities, Greater than 1 year | Security	0
Number of available for sale securities | Security	1
Securities Available For Sale, Less than 12 Months, Fair Value	$ 185
Securities Available For Sale, Less than 12 Months, Unrealized Losses	(8)
Securities Available For Sale, 12 Months or More, Fair Value	0
Securities Available For Sale, 12 Months or More, Unrealized Losses	0
Securities Available For Sale, Fair Value, Total	185
Securities Available For Sale, Unrealized Losses, Total	$ (8)