SECURED FINANCING - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Repurchase Agreements:
Repurchase agreements outstanding	$ 88,554,170	$ 81,115,874	[1]
Repurchase agreements - weighted average effective borrowing rates	2.48%	2.36%
Repurchase agreements - weighted average remaining maturities	72 days	77 days
Maximum repurchase agreements available under counterparty arrangements	$ 1,100,000
Commitments available to be drawn	900,000
Other secured financing long term, amount	4,144,623	$ 4,183,311	[1]
Secured financings and interest rate swaps - collateral held, estimated fair value	98,100,000	90,200,000
Secured financings and interest rate swaps - collateral held, accrued interest	322,700	303,100
Fair value of collateral received in reverse repurchase agreements	$ 538,100	$ 650,000
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	2.99%	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
U.S. Treasury Securities
Repurchase Agreements:
Repurchase agreements outstanding		640,465
1 day
Repurchase Agreements:
Repurchase agreements outstanding	19,083,282	0
1 day | U.S. Treasury Securities
Repurchase Agreements:
Repurchase agreements outstanding		0
2 to 29 days
Repurchase Agreements:
Repurchase agreements outstanding	19,107,841	32,012,641
2 to 29 days | U.S. Treasury Securities
Repurchase Agreements:
Repurchase agreements outstanding		640,465
30 to 59 days
Repurchase Agreements:
Repurchase agreements outstanding	5,220,636	8,164,165
30 to 59 days | U.S. Treasury Securities
Repurchase Agreements:
Repurchase agreements outstanding		0
60 to 89 days
Repurchase Agreements:
Repurchase agreements outstanding	20,762,400	18,689,772
60 to 89 days | U.S. Treasury Securities
Repurchase Agreements:
Repurchase agreements outstanding		0
90 to 119 days
Repurchase Agreements:
Repurchase agreements outstanding	1,867,443	10,067,183
90 to 119 days | U.S. Treasury Securities
Repurchase Agreements:
Repurchase agreements outstanding		0
Over 120 days
Repurchase Agreements:
Repurchase agreements outstanding	$ 22,512,568	12,182,113
Over 120 days | U.S. Treasury Securities
Repurchase Agreements:
Repurchase agreements outstanding		$ 0

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