SECURED FINANCING - Repurchase Agreements - Remaining Maturities, Collateral Types and Weighted Average Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017		Dec. 31, 2016
Repurchase Agreements:
Repurchase agreements outstanding	$ 77,696,343	[1]	$ 65,215,810
Weighted Average Rate	1.61%		1.07%
Percentage of total repurchase agreements with remaining maturity of 1 year	1.00%		7.00%
CRTs
Repurchase Agreements:
Repurchase agreements outstanding	$ 277,971		$ 453,025
Non-Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements outstanding	304,778		720,217
Commercial Loans
Repurchase Agreements:
Repurchase agreements outstanding	385,113		334,867
Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements outstanding	76,703,981		63,707,701
Commercial Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements outstanding	24,500
1 day
Repurchase Agreements:
Repurchase agreements outstanding	$ 0		$ 0
Weighted Average Rate	0.00%		0.00%
1 day | CRTs
Repurchase Agreements:
Repurchase agreements outstanding	$ 0		$ 0
1 day | Non-Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements outstanding	0		0
1 day | Commercial Loans
Repurchase Agreements:
Repurchase agreements outstanding	0		0
1 day | Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements outstanding	0		0
1 day | Commercial Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements outstanding	0
2 to 29 days
Repurchase Agreements:
Repurchase agreements outstanding	$ 33,956,552		$ 30,394,043
Weighted Average Rate	1.69%		0.87%
2 to 29 days | CRTs
Repurchase Agreements:
Repurchase agreements outstanding	$ 263,528		$ 358,972
2 to 29 days | Non-Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements outstanding	253,290		377,366
2 to 29 days | Commercial Loans
Repurchase Agreements:
Repurchase agreements outstanding	0		0
2 to 29 days | Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements outstanding	33,421,609		29,657,705
2 to 29 days | Commercial Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements outstanding	18,125
30 to 59 days
Repurchase Agreements:
Repurchase agreements outstanding	$ 10,828,777		$ 11,694,799
Weighted Average Rate	1.44%		1.10%
30 to 59 days | CRTs
Repurchase Agreements:
Repurchase agreements outstanding	$ 7,229		$ 80,139
30 to 59 days | Non-Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements outstanding	3,658		241,360
30 to 59 days | Commercial Loans
Repurchase Agreements:
Repurchase agreements outstanding	0		0
30 to 59 days | Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements outstanding	10,811,515		11,373,300
30 to 59 days | Commercial Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements outstanding	6,375
60 to 89 days
Repurchase Agreements:
Repurchase agreements outstanding	$ 13,855,787		$ 7,082,232
Weighted Average Rate	1.59%		1.14%
60 to 89 days | CRTs
Repurchase Agreements:
Repurchase agreements outstanding	$ 7,214		$ 13,914
60 to 89 days | Non-Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements outstanding	47,830		101,491
60 to 89 days | Commercial Loans
Repurchase Agreements:
Repurchase agreements outstanding	0		0
60 to 89 days | Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements outstanding	13,800,743		6,966,827
60 to 89 days | Commercial Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements outstanding	0
90 to 119 days
Repurchase Agreements:
Repurchase agreements outstanding	$ 10,128,006		$ 2,063,561
Weighted Average Rate	1.39%		0.89%
90 to 119 days | CRTs
Repurchase Agreements:
Repurchase agreements outstanding	$ 0		$ 0
90 to 119 days | Non-Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements outstanding	0		0
90 to 119 days | Commercial Loans
Repurchase Agreements:
Repurchase agreements outstanding	0		0
90 to 119 days | Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements outstanding	10,128,006		2,063,561
90 to 119 days | Commercial Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements outstanding	0
Over 120 days
Repurchase Agreements:
Repurchase agreements outstanding	$ 8,927,221		$ 13,981,175
Weighted Average Rate	1.77%		1.47%
Over 120 days | CRTs
Repurchase Agreements:
Repurchase agreements outstanding	$ 0		$ 0
Over 120 days | Non-Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements outstanding	0		0
Over 120 days | Commercial Loans
Repurchase Agreements:
Repurchase agreements outstanding	385,113		334,867
Over 120 days | Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements outstanding	8,542,108		$ 13,646,308
Over 120 days | Commercial Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements outstanding	$ 0

[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.