FAIR VALUE MEASUREMENTS - Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands		Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018		Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Assets
Securities (includes pledged assets of $113,624,573 and $87,193,316, respectively) (3)	[1]	$ 119,926,869		$ 92,623,788	[2]
Mortgage servicing rights		425,328	$ 500,745	557,813		$ 599,014	$ 596,378	$ 580,860
Assets transferred or pledged to securitization vehicles		4,211,582		3,833,200	[2]
Interest rate swaps		5,440		48,114
Liabilities
Interest rate swaps		677,295		420,365
Residential Mortgage
Assets
Residential mortgage loans		3,168,105		2,454,637
Level 1
Liabilities
Securitized debt of consolidated VIEs				0
Level 2
Liabilities
Securitized debt of consolidated VIEs				3,347,062
Level 3
Liabilities
Securitized debt of consolidated VIEs				0
Fair Value, Measurements, Recurring
Assets
Agency mortgage-backed securities		118,202,040		90,752,995
Credit risk transfer securities		491,969		552,097
Mortgage servicing rights		425,328		557,813
Assets transferred or pledged to securitization vehicles		3,421,682		3,833,200
Interest rate swaps		5,440		48,114
Other derivatives		69,702		152,389
Total assets		124,910,145		98,575,110
Liabilities
Securitized debt of consolidated VIEs		3,470,168		3,347,062
Interest rate swaps		677,295		420,365
Other derivatives		365,902		469,385
Total liabilities		4,513,365		4,236,812
Fair Value, Measurements, Recurring | Non-Agency Mortgage-backed Securities
Assets
Securities (includes pledged assets of $113,624,573 and $87,193,316, respectively) (3)		1,097,752		1,161,938
Fair Value, Measurements, Recurring | Commercial Securities
Assets
Securities (includes pledged assets of $113,624,573 and $87,193,316, respectively) (3)		135,108		156,758
Fair Value, Measurements, Recurring | Residential Mortgage
Assets
Residential mortgage loans		1,061,124		1,359,806
Fair Value, Measurements, Recurring | Level 1
Assets
Agency mortgage-backed securities		0		0
Credit risk transfer securities		0		0
Mortgage servicing rights		0		0
Assets transferred or pledged to securitization vehicles		0		0
Interest rate swaps		0		0
Other derivatives		20,388		0
Total assets		20,388		0
Liabilities
Securitized debt of consolidated VIEs		0
Interest rate swaps		0		0
Other derivatives		363,165		462,309
Total liabilities		363,165		462,309
Fair Value, Measurements, Recurring | Level 1 | Non-Agency Mortgage-backed Securities
Assets
Securities (includes pledged assets of $113,624,573 and $87,193,316, respectively) (3)		0		0
Fair Value, Measurements, Recurring | Level 1 | Commercial Securities
Assets
Securities (includes pledged assets of $113,624,573 and $87,193,316, respectively) (3)		0		0
Fair Value, Measurements, Recurring | Level 1 | Residential Mortgage
Assets
Residential mortgage loans		0		0
Fair Value, Measurements, Recurring | Level 2
Assets
Agency mortgage-backed securities		118,202,040		90,752,995
Credit risk transfer securities		491,969		552,097
Mortgage servicing rights		0		0
Assets transferred or pledged to securitization vehicles		3,421,682		3,833,200
Interest rate swaps		5,440		48,114
Other derivatives		49,314		152,389
Total assets		124,464,429		98,017,297
Liabilities
Securitized debt of consolidated VIEs		3,470,168
Interest rate swaps		677,295		420,365
Other derivatives		2,737		7,076
Total liabilities		4,150,200		3,774,503
Fair Value, Measurements, Recurring | Level 2 | Non-Agency Mortgage-backed Securities
Assets
Securities (includes pledged assets of $113,624,573 and $87,193,316, respectively) (3)		1,097,752		1,161,938
Fair Value, Measurements, Recurring | Level 2 | Commercial Securities
Assets
Securities (includes pledged assets of $113,624,573 and $87,193,316, respectively) (3)		135,108		156,758
Fair Value, Measurements, Recurring | Level 2 | Residential Mortgage
Assets
Residential mortgage loans		1,061,124		1,359,806
Fair Value, Measurements, Recurring | Level 3
Assets
Agency mortgage-backed securities		0		0
Credit risk transfer securities		0		0
Mortgage servicing rights		425,328		557,813
Assets transferred or pledged to securitization vehicles		0		0
Interest rate swaps		0		0
Other derivatives		0		0
Total assets		425,328		557,813
Liabilities
Securitized debt of consolidated VIEs		0
Interest rate swaps		0		0
Other derivatives		0		0
Total liabilities		0		0
Fair Value, Measurements, Recurring | Level 3 | Non-Agency Mortgage-backed Securities
Assets
Securities (includes pledged assets of $113,624,573 and $87,193,316, respectively) (3)		0		0
Fair Value, Measurements, Recurring | Level 3 | Commercial Securities
Assets
Securities (includes pledged assets of $113,624,573 and $87,193,316, respectively) (3)		0		0
Fair Value, Measurements, Recurring | Level 3 | Residential Mortgage
Assets
Residential mortgage loans		$ 0		$ 0

[1] ⁽³⁾  Excludes $327.9 million and $83.6 million at June 30, 2019 and December 31, 2018, respectively, of non-Agency mortgage-backed securities and $168.0 million and $224.3 million at June 30, 2019 and December 31, 2018, respectively, of commercial mortgage-backed securities in consolidated VIEs pledged as collateral and eliminated from the Company’s Consolidated Statements of Financial Condition.

[2] ⁽¹⁾  Derived from the audited consolidated financial statements at December 31, 2018.