Note Debt Securities Available for Sale (AFS debt securities in a continuous unrealized loss position) (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Less than 12 months Fair Value	$ 4,452,799	$ 4,498,383
Less than 12 months Gross Unrealized Losses	99,242	30,555
12 months or more Fair Value	4,732,034	4,651,571
12 months or more Gross Unrealized Losses	180,841	102,530
Total Fair Value	9,184,833	9,149,954
Total Gross Unrealized Losses	280,083	133,085
US Treasury Securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months Fair Value	2,742,979	2,608,473
Less than 12 months Gross Unrealized Losses	50,265	14,749
12 months or more Fair Value	1,264,938	1,027,066
12 months or more Gross Unrealized Losses	18,140	13,862
Total Fair Value	4,007,917	3,635,539
Total Gross Unrealized Losses	68,405	28,611
Obligations of U.S. Government sponsored entities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months Fair Value	147,211	214,670
Less than 12 months Gross Unrealized Losses	847	1,108
12 months or more Fair Value	381,273	376,807
12 months or more Gross Unrealized Losses	4,582	3,228
Total Fair Value	528,484	591,477
Total Gross Unrealized Losses	5,429	4,336
Obligations of Puerto Rico, States and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than 12 months Fair Value	6,643	6,609
Less than 12 months Gross Unrealized Losses	153	59
12 months or more Fair Value	0	0
12 months or more Gross Unrealized Losses	0	0
Total Fair Value	6,643	6,609
Total Gross Unrealized Losses	153	59
Collateralized Mortgage Obligations - Federal agencies
Debt Securities, Available-for-sale [Line Items]
Less than 12 months Fair Value	195,626	153,336
Less than 12 months Gross Unrealized Losses	4,469	2,110
12 months or more Fair Value	541,559	595,339
12 months or more Gross Unrealized Losses	34,314	24,922
Total Fair Value	737,185	748,675
Total Gross Unrealized Losses	38,783	27,032
Mortgage Backed Securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months Fair Value	1,360,340	1,515,295
Less than 12 months Gross Unrealized Losses	43,508	12,529
12 months or more Fair Value	2,544,264	2,652,359
12 months or more Gross Unrealized Losses	123,805	60,518
Total Fair Value	3,904,604	4,167,654
Total Gross Unrealized Losses	$ 167,313	$ 73,047