Debt Securities Available for Sale - AFS debt securities in a continuous unrealized loss position (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale
Less than 12 months Fair Value	$ 1,191,590	$ 3,396,378
Less than 12 months Gross Unrealized Losses	3,348	7,003
12 months or more Fair Value	3,143,625	6,998,793
12 months or more Gross Unrealized Losses	32,949	202,304
Total Fair Value	4,335,215	10,395,171
Total Gross Unrealized Losses	36,297	209,307
US Treasury Securities
Debt Securities, Available-for-sale
Less than 12 months Fair Value	900,818	3,189,007
Less than 12 months Gross Unrealized Losses	2,376	4,188
12 months or more Fair Value	639,001	2,607,276
12 months or more Gross Unrealized Losses	2,135	36,343
Total Fair Value	1,539,819	5,796,283
Total Gross Unrealized Losses	4,511	40,531
Obligations of U.S. Government sponsored entities
Debt Securities, Available-for-sale
Less than 12 months Fair Value	9,947	14,847
Less than 12 months Gross Unrealized Losses	25	46
12 months or more Fair Value	176,356	318,271
12 months or more Gross Unrealized Losses	380	3,454
Total Fair Value	186,303	333,118
Total Gross Unrealized Losses	405	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	6,931	6,742
12 months or more Gross Unrealized Losses	44	184
Total Fair Value	6,931	6,742
Total Gross Unrealized Losses	44	184
Collateralized Mortgage Obligations - Federal agencies
Debt Securities, Available-for-sale
Less than 12 months Fair Value	103,443	66,652
Less than 12 months Gross Unrealized Losses	467	489
12 months or more Fair Value	343,994	587,869
12 months or more Gross Unrealized Losses	6,495	27,913
Total Fair Value	447,437	654,521
Total Gross Unrealized Losses	6,962	28,402
Mortgage Backed Securities
Debt Securities, Available-for-sale
Less than 12 months Fair Value	177,382	125,872
Less than 12 months Gross Unrealized Losses	480	2,280
12 months or more Fair Value	1,977,343	3,478,635
12 months or more Gross Unrealized Losses	23,895	134,410
Total Fair Value	2,154,725	3,604,507
Total Gross Unrealized Losses	$ 24,375	$ 136,690