FAIR VALUE MEASUREMENTS - Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets
Securities (includes pledged assets of $87,193,316 and $84,752,790, respectively) (2)	[1]	$ 92,623,788	$ 92,563,572
Mortgage servicing rights		557,813	580,860	$ 652,216
Assets transferred or pledged to securitization vehicles		3,833,200	3,306,133
Interest rate swaps		48,114	30,272
Liabilities
Interest rate swaps		420,365	569,129
Residential Mortgage
Assets
Residential mortgage loans		2,454,637	1,438,322
Fair Value, Measurements, Recurring
Assets
Agency mortgage-backed securities		90,752,995	90,551,763
Credit risk transfer securities		552,097	651,764
Mortgage servicing rights		557,813	580,860
Assets transferred or pledged to securitization vehicles		3,833,200	3,306,133
Interest rate swaps		48,114	30,272
Other derivatives		152,389	283,613
Total assets		98,575,110	97,722,996
Liabilities
Securitized debt of consolidated VIEs		3,347,062	2,971,771
Interest rate swaps		420,365	569,129
Other derivatives		469,385	38,725
Total liabilities		4,236,812	3,579,625
Fair Value, Measurements, Recurring | Commercial Securities
Assets
Securities (includes pledged assets of $87,193,316 and $84,752,790, respectively) (2)		156,758	262,751
Fair Value, Measurements, Recurring | Residential Mortgage
Assets
Residential mortgage loans		1,359,806	958,546
Fair Value, Measurements, Recurring | Non-Agency Mortgage-backed Securities
Assets
Securities (includes pledged assets of $87,193,316 and $84,752,790, respectively) (2)		1,161,938	1,097,294
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		0	218,361
Total assets		0	218,361
Liabilities
Securitized debt of consolidated VIEs		0	0
Interest rate swaps		0	0
Other derivatives		462,309	12,285
Total liabilities		462,309	12,285
Fair Value, Measurements, Recurring | Level 1 | Commercial Securities
Assets
Securities (includes pledged assets of $87,193,316 and $84,752,790, respectively) (2)		0	0
Fair Value, Measurements, Recurring | Level 1 | Residential Mortgage
Assets
Residential mortgage loans		0	0
Fair Value, Measurements, Recurring | Level 1 | Non-Agency Mortgage-backed Securities
Assets
Securities (includes pledged assets of $87,193,316 and $84,752,790, respectively) (2)		0	0
Fair Value, Measurements, Recurring | Level 2
Assets
Agency mortgage-backed securities		90,752,995	90,551,763
Credit risk transfer securities		552,097	651,764
Mortgage servicing rights		0	0
Assets transferred or pledged to securitization vehicles		3,833,200	3,306,133
Interest rate swaps		48,114	30,272
Other derivatives		152,389	65,252
Total assets		98,017,297	96,923,775
Liabilities
Securitized debt of consolidated VIEs		3,347,062	2,971,771
Interest rate swaps		420,365	569,129
Other derivatives		7,076	26,440
Total liabilities		3,774,503	3,567,340
Fair Value, Measurements, Recurring | Level 2 | Commercial Securities
Assets
Securities (includes pledged assets of $87,193,316 and $84,752,790, respectively) (2)		156,758	262,751
Fair Value, Measurements, Recurring | Level 2 | Residential Mortgage
Assets
Residential mortgage loans		1,359,806	958,546
Fair Value, Measurements, Recurring | Level 2 | Non-Agency Mortgage-backed Securities
Assets
Securities (includes pledged assets of $87,193,316 and $84,752,790, respectively) (2)		1,161,938	1,097,294
Fair Value, Measurements, Recurring | Level 3
Assets
Agency mortgage-backed securities		0	0
Credit risk transfer securities		0	0
Mortgage servicing rights		557,813	580,860
Assets transferred or pledged to securitization vehicles		0	0
Interest rate swaps		0	0
Other derivatives		0	0
Total assets		557,813	580,860
Liabilities
Securitized debt of consolidated VIEs		0	0
Interest rate swaps		0	0
Other derivatives		0	0
Total liabilities		0	0
Fair Value, Measurements, Recurring | Level 3 | Commercial Securities
Assets
Securities (includes pledged assets of $87,193,316 and $84,752,790, respectively) (2)		0	0
Fair Value, Measurements, Recurring | Level 3 | Residential Mortgage
Assets
Residential mortgage loans		0	0
Fair Value, Measurements, Recurring | Level 3 | Non-Agency Mortgage-backed Securities
Assets
Securities (includes pledged assets of $87,193,316 and $84,752,790, respectively) (2)		$ 0	$ 0

[1] Excludes $83.6 million and $66.3 million at December 31, 2018 and 2017, respectively, of non-Agency mortgage-backed securities and $224.3 million and $0 at December 31, 2018 and December 31, 2017, respectively, of commercial mortgage-backed securities in consolidated VIEs pledged as collateral and eliminated from the Company’s Consolidated Statements of Financial Condition.