SECURED FINANCING - Additional Information (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Repurchase Agreements:
Netted amounts	$ 102,682,104	$ 81,115,874	[1]
Repurchase agreements - weighted average effective borrowing rates	2.24%	2.36%
Repurchase agreements - weighted average remaining maturities	45 days	77 days
Repurchase agreement amount	$ 1,100,000
Remaining capacity of repurchase agreement	895,400
Fair value of collateral received in reverse repurchase agreements	0	$ 650,000
Other secured financing long term, amount	4,466,030	4,183,311	[1]
Secured financings and interest rate swaps - collateral held, estimated fair value	113,700,000	90,200,000
Secured financings and interest rate swaps - collateral held, accrued interest	$ 367,900	$ 303,100
Minimum
Repurchase Agreements:
Maturity period	5 years
Maximum
Repurchase Agreements:
Maturity period	7 years
Maturity Period Beyond Three Years | Period Two | Minimum
Repurchase Agreements:
Maturity period	1 year	1 year
Maturity Period Beyond Three Years | Period Two | Maximum
Repurchase Agreements:
Maturity period	3 years	3 years
FHLB De Moines
Repurchase Agreements:
Debt weighted average interest rate	2.48%	2.78%
Stock held in FHLB	$ 147,900	$ 147,900
FHLB De Moines | Maturity Period Beyond Three Years | Period One
Repurchase Agreements:
Other secured financing long term, amount	90,000
FHLB De Moines | Maturity Period Beyond Three Years | Period Two
Repurchase Agreements:
Other secured financing long term, amount	$ 3,500,000	$ 3,600,000

[1] ⁽¹⁾  Derived from the audited consolidated financial statements at December 31, 2018.