SECURED FINANCING - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017		Dec. 31, 2016	Sep. 30, 2017
Repurchase Agreements:
Repurchase agreements outstanding	$ 77,696,343	[1]	$ 65,215,810
Repurchase agreements - weighted average effective borrowing rates	1.89%		1.64%
Repurchase agreements - weighted average remaining maturities	58 days		96 days
Other secured financing long term, amount	$ 3,837,528	[1]	$ 3,884,708
Secured financings and interest rate swaps - collateral held, estimated fair value	87,000,000		74,300,000
Secured financings and interest rate swaps - collateral held, accrued interest	$ 267,300		229,200
Minimum
Repurchase Agreements:
Other secured financing short term amount, expiration period	3 years
Maximum
Repurchase Agreements:
Other secured financing short term amount, expiration period	8 years
Maturity Period Beyond Three Years
Repurchase Agreements:
Other secured financing long term, amount			$ 3,600,000
Maturity Period Beyond Three Years | Period Two | Minimum
Repurchase Agreements:
Other secured financing short term amount, expiration period	1 year
Maturity Period Beyond Three Years | Period Two | Maximum
Repurchase Agreements:
Other secured financing short term amount, expiration period	3 years
FHLB De Moines
Repurchase Agreements:
Debt weighted average interest rate	1.49%		0.76%
Stock held in FHLB	$ 147,900		$ 145,800
FHLB De Moines | Maturity Period Beyond Three Years
Repurchase Agreements:
Other secured financing short term amount, expiration period			3 years
FHLB De Moines | Maturity Period Beyond Three Years | Period One
Repurchase Agreements:
Other secured financing long term, amount	$ 2,100,000
Other secured financing short term amount, expiration period	3 years
FHLB De Moines | Maturity Period Beyond Three Years | Period Two
Repurchase Agreements:
Other secured financing long term, amount				$ 1,400,000

[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.