INVESTMENT SECURITIES - Available-For-Sale Investments' Fair Value And Gross Unrealized Losses (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Fair Value Less than 12 months	$ 833,777	$ 1,331,998
Unrealized Losses Less than 12 months	6,108	19,090
Fair Value 12 months or more	298,229	43,302
Unrealized Losses 12 months or more	12,653	24,034
Total Fair Value	1,132,006	1,375,300
Total Unrealized Losses	18,761	43,124
Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair Value Less than 12 months	406	408
Unrealized Losses Less than 12 months	4	7
Fair Value 12 months or more	6	0
Unrealized Losses 12 months or more	0	0
Total Fair Value	412	408
Total Unrealized Losses	4	7
Debt Securities [Member] | Puerto Rico Government Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair Value Less than 12 months	0	0
Unrealized Losses Less than 12 months	0	0
Fair Value 12 months or more	2,609	22,609
Unrealized Losses 12 months or more	1,295	15,912
Total Fair Value	2,609	22,609
Total Unrealized Losses	1,295	15,912
Debt Securities [Member] | US States And Political Subdivisions Member [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair Value Less than 12 months	368,367	469,046
Unrealized Losses Less than 12 months	1,815	3,334
Fair Value 12 months or more	89,811	0
Unrealized Losses 12 months or more	593	0
Total Fair Value	458,178	469,046
Total Unrealized Losses	2,408	3,334
Mortgage Backed Securities [Member] | Collateralized Mortgage Obligations Member
Schedule Of Available For Sale Securities [Line Items]
Fair Value Less than 12 months		55,309
Unrealized Losses Less than 12 months		163
Fair Value 12 months or more		0
Unrealized Losses 12 months or more		0
Total Fair Value		55,309
Total Unrealized Losses		163
Mortgage Backed Securities [Member] | Other-mortgage pass-through trust certificates [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair Value Less than 12 months	0	0
Unrealized Losses Less than 12 months	0	0
Fair Value 12 months or more	17,630	20,693
Unrealized Losses 12 months or more	5,990	8,122
Total Fair Value	17,630	20,693
Total Unrealized Losses	5,990	8,122
Mortgage Backed Securities [Member] | Government National Mortgage Association Certificates And Obligations G N M A [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair Value Less than 12 months	10,966	43,388
Unrealized Losses Less than 12 months	56	92
Fair Value 12 months or more	0	0
Unrealized Losses 12 months or more	0	0
Total Fair Value	10,966	43,388
Total Unrealized Losses	56	92
Mortgage Backed Securities [Member] | Federal National Mortgage Association Certificates And Obligations F N M A [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair Value Less than 12 months	320,072	519,008
Unrealized Losses Less than 12 months	2,839	9,667
Fair Value 12 months or more	113,145	0
Unrealized Losses 12 months or more	2,614	0
Total Fair Value	433,217	519,008
Total Unrealized Losses	5,453	9,667
Mortgage Backed Securities [Member] | Federal Home Loan Mortgage Corporation Certificates And Obligations F H L M C [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair Value Less than 12 months	133,966	244,839
Unrealized Losses Less than 12 months	1,394	5,827
Fair Value 12 months or more	75,028	0
Unrealized Losses 12 months or more	2,161	0
Total Fair Value	208,994	244,839
Total Unrealized Losses	$ 3,555	$ 5,827