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Investments in Debt Securities Investments in Debt Securities (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018 | Dec. 31, 2018
Investment [Line Items] |  |  |  |  |  |
Par value of available for sale debt security |  | $ 4,972,691 |  | $ 4,972,691 |  | $ 3,194,441
Investments, Unamortized Premium (Discount) |  | 579,282 |  | 579,282 |  | 590,968
Debt Securities, Available-for-sale, Amortized Cost |  | 5,551,973 |  | 5,551,973 |  | 3,785,409
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 164,566 |  | 164,566 |  | 21,436
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | (2,751) |  | (2,751) |  | (57,381)
Debt Securities, Available-for-sale |  | 5,713,788 |  | 5,713,788 |  | 3,749,464
Proceeds from Sale of Available-for-sale Securities |  | 432,552 | $ 370,993 | 432,552 | $ 526,250 |
Debt Securities, Available-for-sale, Realized Gain (Loss) |  | (10,360) | (12,444) | (10,360) | (16,219) |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Amortized Cost |  | 152,858 |  | 152,858 |  | 39,868
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value |  | 157,963 |  | 157,963 |  | 39,808
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost |  | 285,275 |  | 285,275 |  | 151,041
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value |  | 289,174 |  | 289,174 |  | 152,917
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost |  | 879,723 |  | 879,723 |  | 828,543
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value |  | 914,413 |  | 914,413 |  | 806,015
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost |  | 4,234,117 |  | 4,234,117 |  | 2,765,957
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value |  | 4,352,238 |  | 4,352,238 |  | 2,750,724
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Amortized Cost |  | 5,551,973 |  | 5,551,973 |  | 3,785,409
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Fair Value |  | 5,713,788 |  | 5,713,788 |  | 3,749,464
Agency MBS |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Notional balance for interest only securities |  | 12,861,382 |  | 12,861,382 |  | 13,048,666
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value |  | 148,736 |  | 148,736 |  | 581,440
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | $ 378 |  | $ 378 |  | $ 1,793
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year |  | 14 |  | 14 |  | 28
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value |  | $ 294,704 |  | $ 294,704 |  | $ 1,543,892
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | $ 2,184 |  | $ 2,184 |  | $ 54,260
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year |  | 9 |  | 9 |  | 88
Non-Agency MBS |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Notional balance for interest only securities |  | $ 10,014,205 |  | $ 10,014,205 |  | $ 10,275,494
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value |  | 14,490 |  | 14,490 |  | 70,876
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | $ 140 |  | $ 140 |  | $ 581
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year |  | 4 |  | 4 |  | 22
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value |  | $ 7,117 |  | $ 7,117 |  | $ 46,154
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | $ 51 |  | $ 51 |  | $ 747
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year |  | 8 |  | 8 |  | 19
Residential Mortgage Backed Securities |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Par value of available for sale debt security |  | $ 3,096,152 |  | $ 3,096,152 |  | $ 2,119,495
Investments, Unamortized Premium (Discount) |  | 82,035 |  | 82,035 |  | 56,744
Debt Securities, Available-for-sale, Amortized Cost |  | 3,178,187 |  | 3,178,187 |  | 2,176,239
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 58,076 |  | 58,076 |  | 8,926
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | (2,203) |  | (2,203) |  | (26,286)
Debt Securities, Available-for-sale |  | 3,234,060 |  | 3,234,060 |  | 2,158,879
Commercial Mortgage Backed Securities |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Par value of available for sale debt security |  | 1,876,539 |  | 1,876,539 |  | 1,074,946
Investments, Unamortized Premium (Discount) |  | 14,816 |  | 14,816 |  | 6,481
Debt Securities, Available-for-sale, Amortized Cost |  | 1,891,355 |  | 1,891,355 |  | 1,081,427
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 92,686 |  | 92,686 |  | 6,554
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | (269) |  | (269) |  | (29,550)
Debt Securities, Available-for-sale |  | 1,983,772 |  | 1,983,772 |  | 1,058,431
Interest-Only-Strip |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Investments, Unamortized Premium (Discount) |  | 482,431 |  | 482,431 |  | 527,743
Debt Securities, Available-for-sale, Amortized Cost |  | 482,431 |  | 482,431 |  | 527,743
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 13,804 |  | 13,804 |  | 5,956
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | (279) |  | (279) |  | (1,545)
Debt Securities, Available-for-sale |  | 495,956 |  | 495,956 |  | 532,154
U.S. Treasuries |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Proceeds from Sale of Available-for-sale Securities |  | 0 | 144,461 | 0 | 190,960 |
Debt Securities, Available-for-sale, Realized Gain |  |  |  | 0 |  |
Debt Securities, Available-for-sale, Realized Loss |  | 0 | (4,710) |  | (6,433) |
Agency | Residential Mortgage Backed Securities |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Par value of available for sale debt security |  | 3,095,408 |  | 3,095,408 |  | 2,118,639
Investments, Unamortized Premium (Discount) |  | 82,035 |  | 82,035 |  | 56,744
Debt Securities, Available-for-sale, Amortized Cost |  | 3,177,443 |  | 3,177,443 |  | 2,175,383
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 58,044 |  | 58,044 |  | 8,902
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | (2,184) |  | (2,184) |  | (26,264)
Debt Securities, Available-for-sale |  | $ 3,233,303 |  | $ 3,233,303 |  | $ 2,158,021
Investments, Weighted Average Coupon Rate | [1] | 3.97% |  | 3.97% |  | 3.95%
Proceeds from Sale of Available-for-sale Securities |  | $ 205,493 | 217,837 | $ 205,493 | 217,837 |
Debt Securities, Available-for-sale, Realized Loss |  | (3,953) | (7,785) | (3,953) | (7,785) |
Agency | Commercial Mortgage Backed Securities |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Par value of available for sale debt security |  | 1,875,117 |  | 1,875,117 |  | $ 1,071,906
Investments, Unamortized Premium (Discount) |  | 15,793 |  | 15,793 |  | 8,518
Debt Securities, Available-for-sale, Amortized Cost |  | 1,890,910 |  | 1,890,910 |  | 1,080,424
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 91,994 |  | 91,994 |  | 6,141
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | (269) |  | (269) |  | (29,550)
Debt Securities, Available-for-sale |  | $ 1,982,635 |  | $ 1,982,635 |  | $ 1,057,015
Investments, Weighted Average Coupon Rate | [1] | 2.84% |  | 2.84% |  | 3.22%
Proceeds from Sale of Available-for-sale Securities |  | $ 213,198 | 0 | $ 213,198 | 108,758 |
Debt Securities, Available-for-sale, Realized Gain |  |  | 0 |  |  |
Debt Securities, Available-for-sale, Realized Loss |  | (6,493) |  | (6,493) | (2,052) |
Agency | Interest-Only-Strip |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Investments, Unamortized Premium (Discount) |  | 267,703 |  | 267,703 |  | $ 287,062
Debt Securities, Available-for-sale, Amortized Cost |  | 267,703 |  | 267,703 |  | 287,062
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 8,332 |  | 8,332 |  | 4,281
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | (108) |  | (108) |  | (239)
Debt Securities, Available-for-sale |  | $ 275,927 |  | $ 275,927 |  | $ 291,104
Investments, Weighted Average Coupon Rate | [1] | 0.66% |  | 0.66% |  | 0.55%
Proceeds from Sale of Available-for-sale Securities |  | $ 13,861 | 0 | $ 13,861 | 0 |
Debt Securities, Available-for-sale, Realized Gain |  | 86 | 0 | 86 | 0 |
Non-Agency | Residential Mortgage Backed Securities |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Par value of available for sale debt security |  | 744 |  | 744 |  | $ 856
Investments, Unamortized Premium (Discount) |  | 0 |  | 0 |  | 0
Debt Securities, Available-for-sale, Amortized Cost |  | 744 |  | 744 |  | 856
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 32 |  | 32 |  | 24
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | (19) |  | (19) |  | (22)
Debt Securities, Available-for-sale |  | $ 757 |  | $ 757 |  | $ 858
Investments, Weighted Average Coupon Rate | [1] | 6.75% |  | 6.75% |  | 6.75%
Non-Agency | Commercial Mortgage Backed Securities |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Par value of available for sale debt security |  | $ 1,422 |  | $ 1,422 |  | $ 3,040
Investments, Unamortized Premium (Discount) |  | (977) |  | (977) |  | (2,037)
Debt Securities, Available-for-sale, Amortized Cost |  | 445 |  | 445 |  | 1,003
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 692 |  | 692 |  | 413
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | 0 |  | 0 |  | 0
Debt Securities, Available-for-sale |  | $ 1,137 |  | $ 1,137 |  | $ 1,416
Investments, Weighted Average Coupon Rate | [1] | 5.50% |  | 5.50% |  | 6.47%
Non-Agency | Interest-Only-Strip |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Investments, Unamortized Premium (Discount) |  | $ 214,728 |  | $ 214,728 |  | $ 240,681
Debt Securities, Available-for-sale, Amortized Cost |  | 214,728 |  | 214,728 |  | 240,681
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 5,472 |  | 5,472 |  | 1,675
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | (171) |  | (171) |  | (1,306)
Debt Securities, Available-for-sale |  | $ 220,029 |  | $ 220,029 |  | $ 241,050
Investments, Weighted Average Coupon Rate | [1] | 0.63% |  | 0.63% |  | 0.57%
Proceeds from Sale of Available-for-sale Securities |  | $ 0 | 8,695 | $ 0 | 8,695 |
Debt Securities, Available-for-sale, Realized Gain |  | $ 0 | $ 51 | $ 0 | $ 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 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, 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 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 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, maturing after rolling tenth 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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