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Borrowings under Repurchase Agreements Schedule of Repurchase Agreements by Maturity (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017
Repurchase Agreements [Line Items] |  |  |
Repurchase agreements | [1] | $ 1,589,319 | $ 1,597,206
Debt, Weighted Average Interest Rate |  | 1.76% | 1.47%
Repurchase Agreement Counterparty, Weighted Average Maturity of Agreements |  | 42 days | 43 days
Maturity up to 30 days [Member] |  |  |
Repurchase Agreements [Line Items] |  |  |
Repurchase agreements |  | $ 468,222 | $ 410,628
Debt, Weighted Average Interest Rate |  | 1.67% | 1.41%
Repurchase Agreement Counterparty, Weighted Average Maturity of Agreements |  | 16 days | 15 days
Maturity 31 to 60 days [Member] |  |  |
Repurchase Agreements [Line Items] |  |  |
Repurchase agreements |  | $ 818,835 | $ 906,602
Debt, Weighted Average Interest Rate |  | 1.76% | 1.46%
Repurchase Agreement Counterparty, Weighted Average Maturity of Agreements |  | 45 days | 46 days
Maturity 61 to 90 days [Member] |  |  |
Repurchase Agreements [Line Items] |  |  |
Repurchase agreements |  | $ 302,262 | $ 273,665
Debt, Weighted Average Interest Rate |  | 1.90% | 1.60%
Repurchase Agreement Counterparty, Weighted Average Maturity of Agreements |  | 75 days | 74 days
Maturity 91 to 120 days [Member] |  |  |
Repurchase Agreements [Line Items] |  |  |
Repurchase agreements |  | $ 0 | $ 6,311
Debt, Weighted Average Interest Rate |  | 0.00% | 1.61%
Repurchase Agreement Counterparty, Weighted Average Maturity of Agreements |  | 0 days | 120 days
[1] | In the Company's Consolidated Balance Sheet, all balances associated with the repurchase agreements and financial derivatives are presented on a gross basis.

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- Definition Weighted average maturity (weighted by amount of the agreement) of repurchase agreements with the counterparty, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(m)(1)(i)(B)(iii)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
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- Definition Amount, after the effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities not subject to a master netting arrangement and not elected to be offset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(m)(1)(a)(i)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.13(1)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226008-175313 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=49176635&loc=d3e9760-107771 + Details
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