FAIR VALUE MEASUREMENTS - Estimated Fair Values for All Financial Assets and Liabilities (Detail) - USD ($) $ in Thousands		Dec. 31, 2017		Dec. 31, 2016
Financial assets:
Agency mortgage-backed securities		$ 90,551,763		$ 75,589,873
Credit risk transfer securities		651,764	[1]	724,722
Residential mortgages loans	[2]	1,438,322	[1]	342,289
Mortgage servicing rights		580,860	[1]	652,216
Commercial real estate debt investments	[3]	3,089,108	[1]	4,321,739
Interest rate swaps		30,272	[1]	68,194
Other derivatives		283,613		171,266
Financial liabilities:
Other secured financing		3,837,528	[1]	3,884,708
Securitized debt of consolidated VIEs	[4]	2,971,771	[1]	3,655,802
Interest rate swaps		569,129	[1]	1,443,765
Other derivatives		38,725		86,437
Held-to-maturity debt security, amortized cost				84,500
Held-to-maturity debt security, fair value				87,800
Level 1
Financial assets:
Agency mortgage-backed securities		0		0
Credit risk transfer securities		0		0
Commercial real estate debt investments		0		0
Interest rate swaps		0		0
Other derivatives		218,361		168,209
Financial liabilities:
Interest rate swaps		0		0
Other derivatives		12,285		24,912
Level 2
Financial assets:
Agency mortgage-backed securities		90,551,763		75,589,873
Credit risk transfer securities		651,764		724,722
Commercial real estate debt investments		3,089,108		4,321,739
Interest rate swaps		30,272		68,194
Other derivatives		65,252		3,057
Financial liabilities:
Interest rate swaps		569,129		1,443,765
Other derivatives		26,440		61,525
Level 3
Financial assets:
Agency mortgage-backed securities		0		0
Credit risk transfer securities		0		0
Commercial real estate debt investments		0		0
Interest rate swaps		0		0
Other derivatives		0		0
Financial liabilities:
Interest rate swaps		0		0
Other derivatives		0		0
Carrying Value
Financial assets:
Commercial loans held for sale, net		0		114,425
Carrying Value | Level 1
Financial assets:
Cash and cash equivalents		706,589		1,539,746
Carrying Value | Level 2
Financial assets:
Agency mortgage-backed securities		90,551,763		75,589,873
Credit risk transfer securities		651,764		724,722
Non-Agency mortgage-backed securities		1,097,294		1,401,307
Residential mortgages loans		1,438,322		342,289
Commercial real estate debt investments		3,089,108		4,321,739
Corporate debt		1,011,275		773,274
Interest rate swaps		30,272		68,194
Other derivatives		283,613		171,266
Financial liabilities:
Repurchase agreements		77,696,343		65,215,810
Other secured financing		3,837,528		3,884,708
Securitized debt of consolidated VIEs		2,971,771		3,655,802
Participation sold		0		12,869
Interest rate swaps		569,129		1,443,765
Other derivatives		38,725		86,437
Carrying Value | Level 3
Financial assets:
Mortgage servicing rights		580,860		652,216
Commercial real estate debt and preferred equity, held for investment		1,029,327		970,505
Financial liabilities:
Mortgage payable		309,686		311,636
Fair Value
Financial assets:
Commercial loans held for sale, net		0		114,425
Fair Value | Level 1
Financial assets:
Cash and cash equivalents		706,589		1,539,746
Fair Value | Level 2
Financial assets:
Agency mortgage-backed securities		90,551,763		75,589,873
Credit risk transfer securities		651,764		724,722
Non-Agency mortgage-backed securities		1,097,294		1,401,307
Residential mortgages loans		1,438,322		342,289
Commercial real estate debt investments		3,089,108		4,321,739
Corporate debt		1,014,139		776,310
Interest rate swaps		30,272		68,194
Other derivatives		283,613		171,266
Financial liabilities:
Repurchase agreements		77,697,828		65,256,505
Other secured financing		3,837,595		3,885,430
Securitized debt of consolidated VIEs		2,971,771		3,655,802
Participation sold		0		12,827
Interest rate swaps		569,129		1,443,765
Other derivatives		38,725		86,437
Fair Value | Level 3
Financial assets:
Mortgage servicing rights		580,860		652,216
Commercial real estate debt and preferred equity, held for investment		1,035,095		968,824
Financial liabilities:
Mortgage payable		$ 310,218		$ 312,442

[1] As a result of a change to a clearing organization’s rulebook effective January 3, 2017, beginning with the first quarter 2017 and in subsequent periods the Company is presenting the fair value of centrally cleared interest rate swaps net of variation margin pledged under such transactions. The variation margin was previously reported under cash and cash equivalents and is currently reported as a reduction to interest rate swaps, at fair value. Balances reported prior to the effective date will not be adjusted.

[2] Includes securitized residential mortgage loans of a consolidated VIE carried at fair value of $478.8 million and $165.9 million at December 31, 2017 and 2016, respectively

[3] Includes senior securitized commercial mortgage loans of consolidated VIEs carried at fair value of $2.8 billion and $3.9 billion at December 31, 2017 and 2016, respectively.

[4] Includes securitized debt of consolidated VIEs carried at fair value of $3.0 billion and $3.7 billion at December 31, 2017 and 2016, respectively.