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Investment Securities Available For Sale - Available-For-Sale Securities (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | $ 1,417,724 | $ 829,738 |
Unrealized Gains | 21,031 | 16,449 |
Unrealized Losses | (25,674) | (27,211) |
Investment securities available for sale | 1,413,081 | 818,976 |
Proceeds from sales of investment securities | 107,100 | 208,200 | $ 99,200
Available-for-sale Securities |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Investment securities held in securitization | 47,922 | 43,897 |
Total Agency ARMs |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 93,331 | 114,180 |
Unrealized Gains | 22 | 95 |
Unrealized Losses | (2,281) | (1,430) |
Investment securities available for sale | 91,072 | 112,845 |
Total Agency Fixed Rate |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 1,059,532 | 339,310 |
Unrealized Gains | 669 | 0 |
Unrealized Losses | (12,916) | (10,683) |
Investment securities available for sale | 1,047,285 | 328,627 |
Total Agency IOs |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 41,367 | 97,153 |
Unrealized Gains | 271 | 2,260 |
Unrealized Losses | (10,459) | (14,522) |
Investment securities available for sale | 31,179 | 84,891 |
Total Agency RMBS |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 1,194,230 | 550,643 |
Unrealized Gains | 962 | 2,355 |
Unrealized Losses | (25,656) | (26,635) |
Investment securities available for sale | 1,169,536 | 526,363 |
Non-Agency RMBS |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 100,291 | 162,220 |
Unrealized Gains | 1,852 | 1,218 |
Unrealized Losses | (18) | (154) |
Investment securities available for sale | 102,125 | 163,284 |
U.S. Treasury securities |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 0 | 2,920 |
Unrealized Gains | 0 | 0 |
Unrealized Losses | 0 | (33) |
Investment securities available for sale | 0 | 2,887 |
CMBS |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 123,203 | 113,955 |
Unrealized Gains | 18,217 | 12,876 |
Unrealized Losses | 0 | (389) |
Investment securities available for sale | 141,420 | 126,442 |
CMBS | Available-for-sale Securities |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Investment securities held in securitization | 47,900 | 43,900 |
Freddie Mac | Total Agency ARMs |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 33,623 | 39,138 |
Unrealized Gains | 16 | 24 |
Unrealized Losses | (852) | (528) |
Investment securities available for sale | 32,787 | 38,634 |
Freddie Mac | Total Agency Fixed Rate |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 20,804 | 26,338 |
Unrealized Gains | 0 | 0 |
Unrealized Losses | (736) | (644) |
Investment securities available for sale | 20,068 | 25,694 |
Freddie Mac | Total Agency IOs |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 8,436 | 19,768 |
Unrealized Gains | 19 | 559 |
Unrealized Losses | (2,756) | (3,363) |
Investment securities available for sale | 5,699 | 16,964 |
Fannie Mae | Total Agency ARMs |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 54,958 | 69,031 |
Unrealized Gains | 6 | 71 |
Unrealized Losses | (1,236) | (698) |
Investment securities available for sale | 53,728 | 68,404 |
Fannie Mae | Total Agency Fixed Rate |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 1,038,363 | 312,515 |
Unrealized Gains | 669 | 0 |
Unrealized Losses | (12,174) | (10,035) |
Investment securities available for sale | 1,026,858 | 302,480 |
Fannie Mae | Total Agency IOs |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 11,310 | 27,597 |
Unrealized Gains | 22 | 478 |
Unrealized Losses | (2,989) | (4,777) |
Investment securities available for sale | 8,343 | 23,298 |
Ginnie Mae | Total Agency ARMs |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 4,750 | 6,011 |
Unrealized Gains | 0 | 0 |
Unrealized Losses | (193) | (204) |
Investment securities available for sale | 4,557 | 5,807 |
Ginnie Mae | Total Agency Fixed Rate |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 365 | 457 |
Unrealized Gains | 0 | 0 |
Unrealized Losses | (6) | (4) |
Investment securities available for sale | 359 | 453 |
Ginnie Mae | Total Agency IOs |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 21,621 | 49,788 |
Unrealized Gains | 230 | 1,223 |
Unrealized Losses | (4,714) | (6,382) |
Investment securities available for sale | $ 17,137 | $ 44,629 |

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