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Fair Value Measurements - Summary of Changes in Level 3 Fair Value Category (Detail) - USD ($) $ in Thousands | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2018 | Dec. 31, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value | $ 71,855 | $ 77,630 | $ 72,071 | $ 808,557
Fair Value, Liabilities Measured on Recurring Basis, Change in Unrealized Gain (Loss) | 0 | 0 |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings | 383 | 35,184 |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Gain (Loss) Included in Other Comprehensive Income (Loss) | (270) | (52,508) |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases | 0 | 0 |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances | 0 | 0 |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales | 0 | 0 |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements | (329) | (713,603) |  |
Non-Agency RMBS [Member] |  |  |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value | 0 | 5,309 | 0 | 736,017
Fair Value, Liabilities Measured on Recurring Basis, Change in Unrealized Gain (Loss) | 0 | 0 |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings | 0 | 34,727 |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Gain (Loss) Included in Other Comprehensive Income (Loss) | 0 | (52,138) |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases | 0 | 0 |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales | 0 | 0 |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements | 0 | (713,297) |  |
Residential Mortgage-Backed Securities [Member] |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances | 0 | 0 |  |
Asset-backed Securities [Member] |  |  |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value | 71,855 | 72,321 | 72,071 | 72,540
Fair Value, Liabilities Measured on Recurring Basis, Change in Unrealized Gain (Loss) | 0 | 0 |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings | 383 | 457 |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Gain (Loss) Included in Other Comprehensive Income (Loss) | (270) | (370) |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases | 0 | 0 |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances | 0 | 0 |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales | 0 | 0 |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements | (329) | (306) |  |
Interest Rate Swap [Member] | Interest Rate Swap [Member] |  |  |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability | 54,111 | 50,926 | 47,460 | 61,374
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held | 7,714 | (9,029) |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings | 7,714 | (9,029) |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss | 0 | 0 |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases | 0 | 0 |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Issues | 0 | 0 |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales | 0 | 0 |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements | 1,063 | 1,419 |  |
Credit Derivative [Member] | Credit Derivative [Member] |  |  |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability | (1,077) | (1,018) | (1,459) | (566)
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held | 382 | (452) |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings | 473 | (346) |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss | 0 | 0 |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases | 0 | 0 |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Issues | 0 | 0 |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales | 0 | 0 |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements | 91 | 106 |  |
Derivative [Member] |  |  |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability | 53,034 | 49,908 | 46,001 | 60,808
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held | 8,096 | (9,481) |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings | 8,187 | (9,375) |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss | 0 | 0 |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases | 0 | 0 |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Issues | 0 | 0 |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales | 0 | 0 |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements | 1,154 | 1,525 |  |
Level 3 [Member] |  |  |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability | 7,138,755 | 11,950,737 | 6,930,446 | 12,561,185
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held | 159,875 | (121,468) |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings | 160,349 | (86,178) |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss | 134,688 | 378,151 |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements | 86,728 | 902,421 |  |
Level 3 [Member] | Investments [Member] |  |  |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability | 71,855 | 77,630 | 72,071 | 808,557
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings | 383 | 35,184 |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss | (270) | (52,508) |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements | 329 | 713,603 |  |
Level 3 [Member] | Other Assets [Member] |  |  |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability | 4,148 | 5,621 | 4,516 | 5,979
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held | (368) | (358) |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings | (368) | (358) |  |
Level 3 [Member] | Derivative Financial Instruments, Liabilities [Member] | Derivative Financial Instruments, Liabilities [Member] |  |  |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability | 53,034 | 49,908 | 46,001 | 60,808
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held | 8,096 | (9,481) |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings | 8,187 | (9,375) |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements | 1,154 | 1,525 |  |
Variable Interest Entity, Primary Beneficiary [Member] | Level 3 [Member] | Long-term Debt [Member] |  |  |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability | (224,447) | (2,696,516) | (217,092) | (2,757,688)
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held | (3,053) | 159,985 |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings | (3,053) | 159,985 |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss | (4,302) | (103,877) |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements | 0 | (5,064) |  |
Variable Interest Entity, Primary Beneficiary [Member] | Level 3 [Member] | Loans Receivable [Member] | Loans [Member] |  |  |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability | 4,375,761 | 11,558,331 | 4,287,664 | 11,529,384
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held | 88,281 | (202,588) |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings | 88,281 | (202,588) |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss | 85,061 | 423,892 |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements | 85,245 | 192,357 |  |
Variable Interest Entity, Primary Beneficiary [Member] | Level 3 [Member] | Corporate Debt Securities [Member] | Investments Contract [Member] |  |  |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability | 2,858,404 | 2,955,763 | $ 2,737,286 | $ 2,914,145
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held | 66,919 | (69,026) |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings | 66,919 | (69,026) |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss | $ 54,199 | $ 110,644 |  |

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