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Investments in Debt Securities Investments in Debt Securities (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Investment [Line Items] |  |  |  |  |
Par value of available for sale debt security |  | $ 3,194,441 |  | $ 2,446,491 |
Investments, Unamortized Premium (Discount) |  | 590,968 |  | 736,128 |
Debt Securities, Available-for-sale, Amortized Cost |  | 3,785,409 |  | 3,182,619 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 21,436 |  | 17,185 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | 57,381 |  | 26,285 |
Debt Securities, Available-for-sale |  | 3,749,464 |  | 3,173,519 |
Proceeds from Sale of Available-for-sale Securities |  | 733,064 |  | 1,073,093 | $ 87,818
Debt Securities, Available-for-sale, Realized Gain (Loss) |  | (23,373) |  | (11,530) | (4,238)
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Amortized Cost |  | 39,868 |  | 4,480 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value |  | 39,808 |  | 4,542 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost |  | 151,041 |  | 208,046 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value |  | 152,917 |  | 210,727 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost |  | 828,543 |  | 1,334,795 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value |  | 806,015 |  | 1,326,178 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost |  | 2,765,957 |  | 1,635,298 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value |  | 2,750,724 |  | 1,632,072 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Amortized Cost |  | 3,785,409 |  | 3,182,619 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Fair Value |  | 3,749,464 |  | 3,173,519 |
Agency MBS |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Notional balance for interest only securities |  | 13,048,666 |  | 14,196,122 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value |  | 581,440 |  | 1,293,798 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | $ 1,793 |  | $ 9,769 |
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year |  | 28 |  | 71 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value |  | $ 1,543,892 |  | $ 423,698 |
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | $ 54,260 |  | $ 14,035 |
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year |  | 88 |  | 30 |
Non-Agency MBS |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Notional balance for interest only securities |  | $ 10,275,494 |  | $ 11,006,463 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value |  | 70,876 |  | 51,406 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | $ 581 |  | $ 421 |
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year |  | 22 |  | 11 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value |  | $ 46,154 |  | $ 20,414 |
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | $ 747 |  | $ 323 |
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year |  | 19 |  | 12 |
Residential Mortgage Backed Securities |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Par value of available for sale debt security |  | $ 2,119,495 |  | $ 1,147,623 |
Investments, Unamortized Premium (Discount) |  | 56,744 |  | 46,021 |
Debt Securities, Available-for-sale, Amortized Cost |  | 2,176,239 |  | 1,193,644 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 8,926 |  | 1,667 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | 26,286 |  | 9,959 |
Debt Securities, Available-for-sale |  | 2,158,879 |  | 1,185,352 |
Commercial Mortgage Backed Securities |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Par value of available for sale debt security |  | 1,074,946 |  | 1,150,468 |
Investments, Unamortized Premium (Discount) |  | 6,481 |  | 6,407 |
Debt Securities, Available-for-sale, Amortized Cost |  | 1,081,427 |  | 1,156,875 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 6,554 |  | 5,812 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | 29,550 |  | 13,572 |
Debt Securities, Available-for-sale |  | 1,058,431 |  | 1,149,115 |
Interest-Only-Strip |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Investments, Unamortized Premium (Discount) |  | 527,743 |  | 683,833 |
Debt Securities, Available-for-sale, Amortized Cost |  | 527,743 |  | 683,833 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 5,956 |  | 9,706 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | 1,545 |  | 1,017 |
Debt Securities, Available-for-sale |  | 532,154 |  | 692,522 |
U.S. Treasuries |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Par value of available for sale debt security |  | 0 |  | 148,400 |
Investments, Unamortized Premium (Discount) |  | 0 |  | (133) |
Debt Securities, Available-for-sale, Amortized Cost |  | 0 |  | 148,267 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 0 |  | 0 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | 0 |  | 1,737 |
Debt Securities, Available-for-sale |  | $ 0 |  | $ 146,530 |
Investments, Weighted Average Coupon Rate |  | 0.00% | [1] | 2.13% |
Proceeds from Sale of Available-for-sale Securities |  | $ 248,803 |  | $ 51,797 |
Debt Securities, Available-for-sale, Realized Loss |  | (6,567) |  | (244) |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value |  | 0 |  | 146,530 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | $ 0 |  | $ 1,737 |
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year |  | 0 |  | 1 |
Agency | Residential Mortgage Backed Securities |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Par value of available for sale debt security |  | $ 2,118,639 |  | $ 1,146,553 |
Investments, Unamortized Premium (Discount) |  | 56,744 |  | 46,021 |
Debt Securities, Available-for-sale, Amortized Cost |  | 2,175,383 |  | 1,192,574 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 8,902 |  | 1,626 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | 26,264 |  | 9,939 |
Debt Securities, Available-for-sale |  | $ 2,158,021 |  | $ 1,184,261 |
Investments, Weighted Average Coupon Rate | [1] | 3.95% |  | 3.56% |
Proceeds from Sale of Available-for-sale Securities |  | $ 217,837 |  | $ 716,560 | 54,178
Debt Securities, Available-for-sale, Realized Loss |  | (7,785) |  | (12,392) | (3,010)
Agency | Commercial Mortgage Backed Securities |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Par value of available for sale debt security |  | 1,071,906 |  | 1,123,967 |
Investments, Unamortized Premium (Discount) |  | 8,518 |  | 10,442 |
Debt Securities, Available-for-sale, Amortized Cost |  | 1,080,424 |  | 1,134,409 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 6,141 |  | 3,514 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | 29,550 |  | 13,572 |
Debt Securities, Available-for-sale |  | $ 1,057,015 |  | $ 1,124,351 |
Investments, Weighted Average Coupon Rate | [1] | 3.22% |  | 3.03% |
Proceeds from Sale of Available-for-sale Securities |  | $ 242,029 |  | $ 252,624 |
Debt Securities, Available-for-sale, Realized Loss |  | (9,218) |  | (135) |
Agency | Interest-Only-Strip |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Investments, Unamortized Premium (Discount) |  | 287,062 |  | 375,361 |
Debt Securities, Available-for-sale, Amortized Cost |  | 287,062 |  | 375,361 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 4,281 |  | 5,238 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | 239 |  | 293 |
Debt Securities, Available-for-sale |  | $ 291,104 |  | $ 380,306 |
Investments, Weighted Average Coupon Rate | [1] | 0.55% |  | 0.62% |
Proceeds from Sale of Available-for-sale Securities |  | $ 15,700 |  |  |
Debt Securities, Available-for-sale, Realized Gain |  | 146 |  |  |
Non-Agency | Residential Mortgage Backed Securities |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Par value of available for sale debt security |  | 856 |  | $ 1,070 |
Investments, Unamortized Premium (Discount) |  | 0 |  | 0 |
Debt Securities, Available-for-sale, Amortized Cost |  | 856 |  | 1,070 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 24 |  | 41 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | 22 |  | 20 |
Debt Securities, Available-for-sale |  | $ 858 |  | $ 1,091 |
Investments, Weighted Average Coupon Rate | [1] | 6.75% |  | 6.75% |
Proceeds from Sale of Available-for-sale Securities |  |  |  | $ 16,407 |
Debt Securities, Available-for-sale, Realized Gain |  |  |  | 42 |
Non-Agency | Commercial Mortgage Backed Securities |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Par value of available for sale debt security |  | $ 3,040 |  | 26,501 |
Investments, Unamortized Premium (Discount) |  | (2,037) |  | (4,035) |
Debt Securities, Available-for-sale, Amortized Cost |  | 1,003 |  | 22,466 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 413 |  | 2,298 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | 0 |  | 0 |
Debt Securities, Available-for-sale |  | $ 1,416 |  | $ 24,764 |
Investments, Weighted Average Coupon Rate | [1] | 6.47% |  | 5.47% |
Proceeds from Sale of Available-for-sale Securities |  |  |  | $ 35,705 | 33,640
Debt Securities, Available-for-sale, Realized Loss |  |  |  |  | $ (1,228)
Debt Securities, Available-for-sale, Realized Gain |  |  |  | 1,199 |
Non-Agency | Interest-Only-Strip |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Investments, Unamortized Premium (Discount) |  | $ 240,681 |  | 308,472 |
Debt Securities, Available-for-sale, Amortized Cost |  | 240,681 |  | 308,472 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 1,675 |  | 4,468 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | 1,306 |  | 724 |
Debt Securities, Available-for-sale |  | $ 241,050 |  | $ 312,216 |
Investments, Weighted Average Coupon Rate | [1] | 0.57% |  | 0.61% |
Proceeds from Sale of Available-for-sale Securities |  | $ 8,695 |  |  |
Debt Securities, Available-for-sale, Realized Gain |  | $ 51 |  |  |
[1] | The weighted average coupon (“WAC”) is the gross interest rate of the security weighted by the outstanding principal balance (or by notional balance in the case of an IO security).

X
- Definition The remaining amount of investment premium (discount), net, originally recognized at the purchase date of the instruments that has yet to be amortized (accreted) into earnings. + References No definition available. + Details
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- Definition This item represents the current weighted-average gross interest rate earned on this type of MBS in the investment portfolio as of the period indicated. The gross interest rate of each MBS within the portfolio is weighted based on its current outstanding notional or par balance. The current outstanding notional or par balance for each MBS consists of the total of all principal balances remaining on the mortgage loans underlying each MBS. + References No definition available. + Details
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- Definition Notional balance for interest only securities owned by the Company. Interest only securities represent the right to receive excess interest payments, and not principal cash flows, based on the underlying unpaid principal balance (ie, notional) of the underlying pool of mortgage loans. + References No definition available. + Details
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- Definition Line items represent reporting 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 12 -Sentence Column A + Details
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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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- Definition Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + 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 No definition available. + Details
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- Definition Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in next rolling fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date. + References No definition available. + Details
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- Definition Fair value of investment in debt security, measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in next rolling fiscal year through fifth year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date. + References No definition available. + Details
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- Definition Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62586-112803 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27198-111563 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62586-112803 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27198-111563 + Details
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