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Investment Securities Available For Sale - Unrealized Losses in OCI (Details) - USD ($) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract] |  |  |
Other-than-temporary impairment loss | $ 0 | $ 0 | $ 0
Schedule of Available-for-sale Securities [Line Items] |  |  |
Carrying Value Less Than 12 Months | 511,313,000 | 112,880,000 |
Gross Unrealized Losses Less Than 12 Months | (1,807,000) | (1,679,000) |
Carrying Value Greater Than 12 Months | 343,156,000 | 329,070,000 |
Gross Unrealized Losses Greater Than 12 Months | (13,408,000) | (10,973,000) |
Carrying Value Total | 854,469,000 | 441,950,000 |
Gross Unrealized Losses Total | (15,215,000) | (12,652,000) |
Agency RMBS |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Carrying Value Less Than 12 Months | 511,313,000 | 96,357,000 |
Gross Unrealized Losses Less Than 12 Months | (1,807,000) | (1,290,000) |
Carrying Value Greater Than 12 Months | 342,963,000 | 328,474,000 |
Gross Unrealized Losses Greater Than 12 Months | (13,390,000) | (10,819,000) |
Carrying Value Total | 854,276,000 | 424,831,000 |
Gross Unrealized Losses Total | (15,197,000) | (12,109,000) |
Non-Agency RMBS |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Carrying Value Less Than 12 Months | 0 | 0 |
Gross Unrealized Losses Less Than 12 Months | 0 | 0 |
Carrying Value Greater Than 12 Months | 193,000 | 596,000 |
Gross Unrealized Losses Greater Than 12 Months | (18,000) | (154,000) |
Carrying Value Total | 193,000 | 596,000 |
Gross Unrealized Losses Total | $ (18,000) | (154,000) |
CMBS |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Carrying Value Less Than 12 Months |  | 16,523,000 |
Gross Unrealized Losses Less Than 12 Months |  | (389,000) |
Carrying Value Greater Than 12 Months |  | 0 |
Gross Unrealized Losses Greater Than 12 Months |  | 0 |
Carrying Value Total |  | 16,523,000 |
Gross Unrealized Losses Total |  | $ (389,000) |

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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 + Details
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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 + Details
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- Definition This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 + Details
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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 + Details
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- Definition This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 + Details
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- Definition This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for twelve months or longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 + Details
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- Definition The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8B -URI http://asc.fasb.org/extlink&oid=82897076&loc=SL6284393-111563 + Details
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