FAIR VALUE - Assets - Assets and Liabilities Recorded at Fair Value on Recurring Basis (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	$ 3,930,340,000		$ 3,930,340,000		$ 3,022,174,000
Derivative Asset, Fair Value, Gross Asset	16,871,000		16,871,000		10,365,000
Fair Value Assets Transfers Amount	0	$ 0	0	$ 0
Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	10,693,000		10,693,000		10,302,000
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	3,930,340,000		3,930,340,000		3,022,174,000
Estimate of Fair Value, Fair Value Disclosure [Member] | Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	8,102,000		8,102,000		9,500,000
Estimate of Fair Value, Fair Value Disclosure [Member] | Interest Rate Swaption [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	3,194,000		3,194,000
Estimate of Fair Value, Fair Value Disclosure [Member] | TBA Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	2,984,000		2,984,000		(1,117,000)
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	0		0		0
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	0		0		0
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Interest Rate Swaption [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	0		0
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | TBA Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	0		0		0
Fair Value, Inputs, Level 2 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	3,930,340,000		3,930,340,000		3,022,174,000
Fair Value, Inputs, Level 2 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	8,102,000		8,102,000		9,500,000
Fair Value, Inputs, Level 2 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Interest Rate Swaption [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	3,194,000		3,194,000
Fair Value, Inputs, Level 2 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | TBA Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	2,984,000		2,984,000		(1,117,000)
Fair Value, Inputs, Level 3 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	$ 0		$ 0		0
Fair Value, Inputs, Level 3 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset					0
Fair Value, Inputs, Level 3 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | TBA Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset					$ 0