SECURED FINANCING - Additional Information (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Repurchase Agreements:
Repurchase agreements	$ 75,760,655	$ 77,696,343	[1]
Repurchase agreements - weighted average effective borrowing rates	1.91%	1.89%
Repurchase agreements - weighted average remaining maturities	71 days	58 days
Other secured financing long term, amount	$ 3,760,487	$ 3,837,528	[1]
Secured financings and interest rate swaps - collateral held, estimated fair value	85,000,000	87,000,000
Secured financings and interest rate swaps - collateral held, accrued interest	270,600	$ 267,300
Fair value of collateral received	$ 261,100
Minimum
Repurchase Agreements:
Other secured financing amount, long term, expiration period	2 years
Maximum
Repurchase Agreements:
Other secured financing amount, long term, expiration period	8 years
FHLB De Moines
Repurchase Agreements:
Debt weighted average interest rate	2.40%	1.49%
Stock held in FHLB	$ 147,900	$ 147,900
Maturity Period Beyond Three Years
Repurchase Agreements:
Other secured financing long term, amount		$ 2,100,000
Other secured financing amount, long term, expiration period		3 years
Maturity Period Beyond Three Years | Minimum
Repurchase Agreements:
Other secured financing amount, long term, expiration period	1 year	1 year
Maturity Period Beyond Three Years | Maximum
Repurchase Agreements:
Other secured financing amount, long term, expiration period	3 years	3 years
Maturity Period Beyond Three Years | FHLB De Moines | Debt Instrument, Maturity, Period Two
Repurchase Agreements:
Other secured financing long term, amount	$ 3,600,000	$ 1,400,000

[1] Derived from the audited consolidated financial statements at December 31, 2017.