Debt Securities Available for Sale - AFS debt securities in a continuous unrealized loss position (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale
Less than 12 months Fair Value	$ 748,566	$ 3,396,378
Less than 12 months Gross Unrealized Losses	123	7,003
12 months or more Fair Value	4,628,120	6,998,793
12 months or more Gross Unrealized Losses	58,867	202,304
Total Fair Value	5,376,686	10,395,171
Total Gross Unrealized Losses	58,990	209,307
US Treasury Securities
Debt Securities, Available-for-sale
Less than 12 months Fair Value	746,094	3,189,007
Less than 12 months Gross Unrealized Losses	102	4,188
12 months or more Fair Value	1,094,470	2,607,276
12 months or more Gross Unrealized Losses	4,264	36,343
Total Fair Value	1,840,564	5,796,283
Total Gross Unrealized Losses	4,366	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	230,782	318,271
12 months or more Gross Unrealized Losses	900	3,454
Total Fair Value	230,782	333,118
Total Gross Unrealized Losses	900	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	6,878	6,742
12 months or more Gross Unrealized Losses	97	184
Total Fair Value	6,878	6,742
Total Gross Unrealized Losses	97	184
Collateralized Mortgage Obligations - Federal agencies
Debt Securities, Available-for-sale
Less than 12 months Fair Value	0	66,652
Less than 12 months Gross Unrealized Losses	0	489
12 months or more Fair Value	455,372	587,869
12 months or more Gross Unrealized Losses	10,153	27,913
Total Fair Value	455,372	654,521
Total Gross Unrealized Losses	10,153	28,402
Mortgage Backed Securities
Debt Securities, Available-for-sale
Less than 12 months Fair Value	2,472	125,872
Less than 12 months Gross Unrealized Losses	21	2,280
12 months or more Fair Value	2,840,618	3,478,635
12 months or more Gross Unrealized Losses	43,453	134,410
Total Fair Value	2,843,090	3,604,507
Total Gross Unrealized Losses	$ 43,474	$ 136,690