Note Debt Securities Available for Sale (AFS debt securities in a continuous unrealized loss position) (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale
Less than 12 months Fair Value	$ 1,495,464	$ 3,396,378
Less than 12 months Gross Unrealized Losses	301	7,003
12 months or more Fair Value	6,815,263	6,998,793
12 months or more Gross Unrealized Losses	127,443	202,304
Total Fair Value	8,310,727	10,395,171
Total Gross Unrealized Losses	127,744	209,307
US Treasury Securities
Debt Securities, Available-for-sale
Less than 12 months Fair Value	1,473,943	3,189,007
Less than 12 months Gross Unrealized Losses	76	4,188
12 months or more Fair Value	2,604,306	2,607,276
12 months or more Gross Unrealized Losses	20,408	36,343
Total Fair Value	4,078,249	5,796,283
Total Gross Unrealized Losses	20,484	40,531
Obligations of U.S. Government sponsored entities
Debt Securities, Available-for-sale
Less than 12 months Fair Value	0	14,847
Less than 12 months Gross Unrealized Losses	0	46
12 months or more Fair Value	263,499	318,271
12 months or more Gross Unrealized Losses	2,160	3,454
Total Fair Value	263,499	333,118
Total Gross Unrealized Losses	2,160	3,500
Obligations of Puerto Rico, States and political subdivisions
Debt Securities, Available-for-sale
Less than 12 months Fair Value	0	0
Less than 12 months Gross Unrealized Losses	0	0
12 months or more Fair Value	6,809	6,742
12 months or more Gross Unrealized Losses	166	184
Total Fair Value	6,809	6,742
Total Gross Unrealized Losses	166	184
Collateralized Mortgage Obligations - Federal agencies
Debt Securities, Available-for-sale
Less than 12 months Fair Value	3,447	66,652
Less than 12 months Gross Unrealized Losses	5	489
12 months or more Fair Value	480,426	587,869
12 months or more Gross Unrealized Losses	21,000	27,913
Total Fair Value	483,873	654,521
Total Gross Unrealized Losses	21,005	28,402
Mortgage Backed Securities
Debt Securities, Available-for-sale
Less than 12 months Fair Value	18,074	125,872
Less than 12 months Gross Unrealized Losses	220	2,280
12 months or more Fair Value	3,460,223	3,478,635
12 months or more Gross Unrealized Losses	83,709	134,410
Total Fair Value	3,478,297	3,604,507
Total Gross Unrealized Losses	$ 83,929	$ 136,690