Debt Securities Available for Sale - Weighted average yield and contractual maturities of debt securities AFS (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Debt securities available-for-sale, at fair value	$ 16,734,722	$ 13,300,184
Gross Unrealized Gains	133,050	30,929
Gross Unrealized Losses	58,990	209,307
Amortized Cost	16,660,662	13,478,562
Amortized cost
Amortized Cost	16,660,662	13,478,562
Gross unrealized gains
Gross unrealized gains	133,050	30,929
Gross unrealized loss
Gross unrealized losses	$ 58,990	$ 209,307
Weighted average yield
Weighted Average Yield	2.30%	2.25%
US Treasury Securities
Amortized cost
Amortized cost - Within 1 year	$ 6,143,915	$ 3,565,571
Amortized cost - After 1 to 5 years	4,265,188	4,483,741
Amortized cost - After 5 to 10 years	7,064	245,891
Amortized Cost	10,416,167	8,295,203
Gross unrealized gains
Gross unrealized gain - Within 1 year	3,097	108
Gross unrealized gain - After 1 to 5 years	76,531	13,647
Gross unrealized gain - After 5 to 10 years	235	3,770
Gross unrealized gains	79,863	17,525
Gross unrealized loss
Gross unrealized losses - Within 1 year	2,697	5,319
Gross unrealized losses - After 1 to 5 years	1,669	35,213
Gross unrealized losses - After 5 to 10 years	0	0
Gross unrealized losses	4,366	40,532
Fair Value
Fair value - Within 1 year	6,144,315	3,560,360
Fair value - After 1 to 5 years	4,340,050	4,462,175
Fair value - After 5 to 10 years	7,299	249,661
Fair Value	$ 10,491,664	$ 8,272,196
Weighted average yield
Weighted average yield - Within 1 year	2.08%	2.10%
Weighted average yield - After 1 to 5 years	2.34%	2.25%
Weighted average yield - After 5 to 10 years	2.41%	2.84%
Weighted Average Yield	2.19%	2.21%
Obligations of U.S. Government sponsored entities
Amortized cost
Amortized cost - Within 1 year	$ 186,092	$ 212,951
Amortized cost - After 1 to 5 years	60,752	123,857
Amortized Cost	246,844	336,808
Gross unrealized gains
Gross unrealized gain - Within 1 year	1	0
Gross unrealized gain - After 1 to 5 years	2	1
Gross unrealized gains	3	1
Gross unrealized loss
Gross unrealized losses - Within 1 year	527	1,406
Gross unrealized losses - After 1 to 5 years	373	2,094
Gross unrealized losses	900	3,500
Fair Value
Fair value - Within 1 year	185,566	211,545
Fair value - After 1 to 5 years	60,381	121,764
Fair Value	$ 245,947	$ 333,309
Weighted average yield
Weighted average yield - Within 1 year	1.45%	1.44%
Weighted average yield - After 1 to 5 years	1.54%	1.51%
Weighted Average Yield	1.47%	1.47%
Obligations of Puerto Rico, States and political subdivisions
Amortized cost
Amortized cost - Within 1 year	$ 6,975
Amortized cost - After 1 to 5 years		$ 6,926
Amortized Cost	6,975	6,926
Gross unrealized gains
Gross unrealized gain - Within 1 year	0
Gross unrealized gain - After 1 to 5 years		0
Gross unrealized gains	0	0
Gross unrealized loss
Gross unrealized losses - Within 1 year	97
Gross unrealized losses - After 1 to 5 years		184
Gross unrealized losses	97	184
Fair Value
Fair value - Within 1 year	6,878
Fair value - After 1 to 5 years		6,742
Fair Value	$ 6,878	$ 6,742
Weighted average yield
Weighted average yield - Within 1 year	0.00%
Weighted average yield - After 1 to 5 years		0.70%
Weighted Average Yield	0.00%	0.70%
Collateralized Mortgage Obligations - Federal agencies
Amortized cost
Amortized cost - After 1 to 5 years	$ 902	$ 749
Amortized cost - After 5 to 10 years	98,268	115,744
Amortized cost - After 10 years	577,850	638,995
Amortized Cost	677,020	755,488
Gross unrealized gains
Gross unrealized gain - After 1 to 5 years	1	0
Gross unrealized gain - After 5 to 10 years	22	1
Gross unrealized gain - After 10 years	4,278	1,584
Gross unrealized gains	4,301	1,585
Gross unrealized loss
Gross unrealized losses - After 1 to 5 years	2	7
Gross unrealized losses - After 5 to 10 years	1,851	4,715
Gross unrealized losses - After 10 years	8,300	23,680
Gross unrealized losses	10,153	28,402
Fair Value
Fair value - After 1 to 5 years	901	742
Fair value - After 5 to 10 years	96,439	111,030
Fair value - After 10 years	573,828	616,899
Fair Value	$ 671,168	$ 728,671
Weighted average yield
Weighted average yield - After 1 to 5 years	2.01%	1.92%
Weighted average yield - After 5 to 10 years	1.66%	1.71%
Weighted average yield - After 10 years	2.10%	2.10%
Weighted Average Yield	2.04%	2.04%
Mortgage Backed Securities
Amortized cost
Amortized cost - Within 1 year	$ 69	$ 431
Amortized cost - After 1 to 5 years	42,319	6,762
Amortized cost - After 5 to 10 years	301,619	365,727
Amortized cost - After 10 years	4,969,253	3,710,731
Amortized Cost	5,313,260	4,083,651
Gross unrealized gains
Gross unrealized gain - Within 1 year	1	4
Gross unrealized gain - After 1 to 5 years	841	43
Gross unrealized gain - After 5 to 10 years	1,755	1,090
Gross unrealized gain - After 10 years	46,278	10,679
Gross unrealized gains	48,875	11,816
Gross unrealized loss
Gross unrealized losses - Within 1 year	0	0
Gross unrealized losses - After 1 to 5 years	1	1
Gross unrealized losses - After 5 to 10 years	2,148	8,499
Gross unrealized losses - After 10 years	41,325	128,189
Gross unrealized losses	43,474	136,689
Fair Value
Fair value - Within 1 year	70	435
Fair value - After 1 to 5 years	43,159	6,804
Fair value - After 5 to 10 years	301,226	358,318
Fair value - After 10 years	4,974,206	3,593,221
Fair Value	$ 5,318,661	$ 3,958,778
Weighted average yield
Weighted average yield - Within 1 year	2.37%	4.30%
Weighted average yield - After 1 to 5 years	3.32%	2.74%
Weighted average yield - After 5 to 10 years	2.04%	2.19%
Weighted average yield - After 10 years	2.61%	2.45%
Weighted Average Yield	2.59%	2.43%
Other
Amortized cost
Amortized cost - After 5 to 10 years	$ 396	$ 486
Amortized Cost	396	486
Gross unrealized gains
Gross unrealized gain - After 5 to 10 years	8	2
Gross unrealized gains	8	2
Gross unrealized loss
Gross unrealized losses - After 5 to 10 years	0	0
Gross unrealized losses	0	0
Fair Value
Fair value - After 5 to 10 years	404	488
Fair Value	$ 404	$ 488
Weighted average yield
Weighted average yield - After 5 to 10 years	3.62%	3.62%
Weighted Average Yield	3.62%	3.62%