Note Debt Securities Available for Sale (AFS debt securities in a continuous unrealized loss position) (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Less than 12 months Fair Value	$ 4,769,329	$ 4,498,383
Less than 12 months Gross Unrealized Losses	85,261	30,555
12 months or more Fair Value	4,775,375	4,651,571
12 months or more Gross Unrealized Losses	162,707	102,530
Total Fair Value	9,544,704	9,149,954
Total Gross Unrealized Losses	247,968	133,085
US Treasury Securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months Fair Value	2,782,548	2,608,473
Less than 12 months Gross Unrealized Losses	38,901	14,749
12 months or more Fair Value	1,332,915	1,027,066
12 months or more Gross Unrealized Losses	18,090	13,862
Total Fair Value	4,115,463	3,635,539
Total Gross Unrealized Losses	56,991	28,611
Obligations of U.S. Government sponsored entities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months Fair Value	228,285	214,670
Less than 12 months Gross Unrealized Losses	1,532	1,108
12 months or more Fair Value	351,070	376,807
12 months or more Gross Unrealized Losses	3,987	3,228
Total Fair Value	579,355	591,477
Total Gross Unrealized Losses	5,519	4,336
Obligations of Puerto Rico, States and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than 12 months Fair Value	6,613	6,609
Less than 12 months Gross Unrealized Losses	119	59
12 months or more Fair Value	0	0
12 months or more Gross Unrealized Losses	0	0
Total Fair Value	6,613	6,609
Total Gross Unrealized Losses	119	59
Collateralized Mortgage Obligations - Federal agencies
Debt Securities, Available-for-sale [Line Items]
Less than 12 months Fair Value	208,888	153,336
Less than 12 months Gross Unrealized Losses	3,646	2,110
12 months or more Fair Value	578,882	595,339
12 months or more Gross Unrealized Losses	32,169	24,922
Total Fair Value	787,770	748,675
Total Gross Unrealized Losses	35,815	27,032
Mortgage Backed Securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months Fair Value	1,542,995	1,515,295
Less than 12 months Gross Unrealized Losses	41,063	12,529
12 months or more Fair Value	2,512,508	2,652,359
12 months or more Gross Unrealized Losses	108,461	60,518
Total Fair Value	4,055,503	4,167,654
Total Gross Unrealized Losses	$ 149,524	$ 73,047