Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

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Derivative, Variable Interest Rate, Term

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Represents the weighted average interest rate paid related to derivatives.

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Represents the weighted average interest rate received related to derivatives.

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Average fixed interest rate related to the group of interest rate derivatives.

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Average remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Average variable interest rate related to the group of interest rate derivatives.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of decrease in the fair value of derivatives recognized in the income statement.

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Period remaining until the derivative contract matures, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of the cost of borrowed funds accounted for as interest expense.

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The carrying amount, as of the date of the latest financial statement presented, of all assets that are pledged as collateral for the payment of the related debt obligation primarily arising from secured borrowings, repurchase agreements or other current or potential obligations, where the secured party does not have the right by contract or custom to sell or re-pledge the collateral and therefore are not reclassified and separately reported in the statement of financial position.

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(Swaps) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
(Swaps) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Interest expense attributable to Swaps	$ 70,944	$ 58,878	$ 51,810	$ 50,554	$ 48,495	$ 49,275	$ 49,022	$ 50,349	$ 232,186	$ 197,141	$ 193,355
Interest Rate Contract
Derivative [Line Items]
Assets Pledged	32,803				28,840				32,803	28,840
Interest Rate Contract \| Agency MBS, at fair value
Derivative [Line Items]
Assets Pledged	2,735				21,756				2,735	21,756
Interest Rate Contract \| Restricted cash
Derivative [Line Items]
Assets Pledged	30,068				7,084				30,068	7,084
Swaps, at fair value
Derivative [Line Items]
Aggregate notional amount of derivatives	$ 3,217,000				$ 2,550,000				$ 3,217,000	$ 2,550,000
Average maturity term of swaps									32 months
Weighted average fixed-pay rate	2.42%				2.04%				2.42%	2.04%
Weighted Average Variable Interest Rate	2.56%				1.50%				2.56%	1.50%
Interest expense attributable to Swaps									$ 3,780	$ 24,524	$ 40,898
Weighted average Swap rate paid									2.12%	1.98%	1.82%
Weighted average Swap rate received									1.96%	1.07%	0.48%
Swaps, at fair value \| Within 30 days
Derivative [Line Items]
Aggregate notional amount of derivatives	$ 0				$ 0				$ 0	$ 0
Weighted average fixed-pay rate	0.00%				0.00%				0.00%	0.00%
Weighted Average Variable Interest Rate	0.00%				0.00%				0.00%	0.00%
Swaps, at fair value \| Over 30 days to 3 months
Derivative [Line Items]
Aggregate notional amount of derivatives	$ 100,000				$ 0				$ 100,000	$ 0
Weighted average fixed-pay rate	1.71%				0.00%				1.71%	0.00%
Weighted Average Variable Interest Rate	2.50%				0.00%				2.50%	0.00%
Swaps, at fair value \| Over 3 months to 6 months
Derivative [Line Items]
Aggregate notional amount of derivatives	$ 100,000				$ 50,000				$ 100,000	$ 50,000
Weighted average fixed-pay rate	1.71%				1.45%				1.71%	1.45%
Weighted Average Variable Interest Rate	2.50%				1.56%				2.50%	1.56%
Swaps, at fair value \| Over 6 months to 12 months
Derivative [Line Items]
Aggregate notional amount of derivatives	$ 0				$ 500,000				$ 0	$ 500,000
Weighted average fixed-pay rate	0.00%				1.50%				0.00%	1.50%
Weighted Average Variable Interest Rate	0.00%				1.46%				0.00%	1.46%
Swaps, at fair value \| Over 12 months to 24 months
Derivative [Line Items]
Aggregate notional amount of derivatives	$ 1,630,000				$ 200,000				$ 1,630,000	$ 200,000
Weighted average fixed-pay rate	2.27%				1.71%				2.27%	1.71%
Weighted Average Variable Interest Rate	2.50%				1.54%				2.50%	1.54%
Swaps, at fair value \| Over 24 months to 36 months
Derivative [Line Items]
Aggregate notional amount of derivatives	$ 822,000				$ 1,500,000				$ 822,000	$ 1,500,000
Weighted average fixed-pay rate	2.57%				2.22%				2.57%	2.22%
Weighted Average Variable Interest Rate	2.64%				1.51%				2.64%	1.51%
Swaps, at fair value \| Over 36 months to 48 months
Derivative [Line Items]
Aggregate notional amount of derivatives	$ 0				$ 200,000				$ 0	$ 200,000
Weighted average fixed-pay rate	0.00%				2.20%				0.00%	2.20%
Weighted Average Variable Interest Rate	0.00%				1.53%				0.00%	1.53%
Swaps, at fair value \| Over 48 months to 60 months
Derivative [Line Items]
Aggregate notional amount of derivatives	$ 395,000				$ 0				$ 395,000	$ 0
Weighted average fixed-pay rate	2.88%				0.00%				2.88%	0.00%
Weighted Average Variable Interest Rate	2.63%				0.00%				2.63%	0.00%
Swaps, at fair value \| Over 60 months to 72 months
Derivative [Line Items]
Aggregate notional amount of derivatives	$ 0				$ 100,000				$ 0	$ 100,000
Weighted average fixed-pay rate	0.00%				2.75%				0.00%	2.75%
Weighted Average Variable Interest Rate	0.00%				1.50%				0.00%	1.50%
Swaps, at fair value \| Over 72 months to 84 months
Derivative [Line Items]
Aggregate notional amount of derivatives	$ 0				$ 0				$ 0	$ 0
Weighted average fixed-pay rate	0.00%				0.00%				0.00%	0.00%
Weighted Average Variable Interest Rate	0.00%				0.00%				0.00%	0.00%
Swaps, at fair value \| Over 84 months
Derivative [Line Items]
Aggregate notional amount of derivatives	$ 170,000				$ 0				$ 170,000	$ 0
Weighted average fixed-pay rate	3.00%				0.00%				3.00%	0.00%
Weighted Average Variable Interest Rate	2.66%				0.00%				2.66%	0.00%
Swaps, at fair value \| Hedging
Derivative [Line Items]
Aggregate notional amount of derivatives	$ 3,200,000								$ 3,200,000
Swaps, at fair value \| Minimum \| LIBOR
Derivative [Line Items]
Derivative, variable interest rate, term									1 month
Swaps, at fair value \| Maximum
Derivative [Line Items]
Maximum maturity term of swaps									120 months
Swaps, at fair value \| Maximum \| LIBOR
Derivative [Line Items]
Derivative, variable interest rate, term									3 months
Swaps \| Non-Hedging
Derivative [Line Items]
Loss on derivative									$ 9,600