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Fair Value Measurements - Information about Valuation Inputs for Fixed Income Securities Classified as Level 3 (Detail) - Successor [Member] - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended | 12 Months Ended |  |
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Jun. 30, 2018 | Dec. 31, 2017 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Mar. 31, 2018 | Mar. 31, 2017
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |  |  |  |  |  |
Available-for-sale Securities | $ 3,993,015 | $ 5,309,161 |  | $ 3,993,015 |  | $ 5,309,161 |  |  |
Ambac insured Re-remic [Member] |  |  |  |  |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |  |  |  |  |  |
Coupon rate |  | 2.05% |  |  |  | 2.05% |  |  |
Fair Value Inputs Maturity |  |  |  |  |  | 7 months 24 days |  |  |
Yield |  | 0.00% |  |  |  | 0.00% |  |  |
Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |  |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |  |  |  |  |  |
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held |  |  |  |  |  | $ (71,072) | $ 266,629 |  |
Fair Value, Inputs, Level 3 [Member] |  |  |  |  |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |  |  |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability | 11,110,155 | $ 12,561,185 | $ 12,044,498 | 11,110,155 | $ 12,044,498 | 12,561,185 | 11,369,112 | $ 11,950,737 | $ 11,588,457
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings | 37,791 |  | 90,285 | (48,387) | 343,534 |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss | (691,566) |  | 412,299 | (313,415) | 602,225 |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases |  |  | 25,930 | 0 | 35,781 |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales |  |  |  | 0 | (79,319) |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements | 181,498 |  | 72,473 | 1,083,919 | (248,913) |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Transfers Into Level Three |  |  |  |  | 22,078 |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Transfers Out Of Level Three | 5,309 |  |  | 5,309 |  |  |  |  |
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held | 37,418 |  | 23,931 | (84,050) | 269,852 |  |  |  |
Fair Value, Inputs, Level 3 [Member] | RMBS Cash Flows Consisting of Deferred Amounts [Member] |  |  |  |  |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |  |  |  |  |  |
Available-for-sale Securities | 0 | 709,950 |  | 0 |  | 709,950 |  |  |
Fair Value, Inputs, Level 3 [Member] | Ambac insured Re-remic [Member] |  |  |  |  |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |  |  |  |  |  |
Available-for-sale Securities |  | 26,067 |  |  |  | 26,067 |  |  |
Fair Value, Inputs, Level 3 [Member] | Corporate Obligations [Member] |  |  |  |  |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |  |  |  |  |  |
Available-for-sale Securities | 0 | 0 |  | 0 |  | 0 |  |  |
Fair Value, Inputs, Level 3 [Member] | Variable Interest Entity, Primary Beneficiary [Member] | Corporate Obligations [Member] |  |  |  |  |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |  |  |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability |  |  |  |  |  |  | 2,622,566 |  |
Available-for-sale Securities | 2,756,924 | 2,914,145 |  | 2,756,924 |  | 2,914,145 |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings |  |  |  |  | (31,454) |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss |  |  |  |  | 147,131 |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases |  |  |  |  | 0 |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales |  |  |  |  | 0 |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements |  |  |  |  | (15,927) |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Transfers Into Level Three |  |  |  |  | 0 |  |  |  |
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held |  |  |  |  | (31,454) |  |  |  |
Derivative [Member] |  |  |  |  |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |  |  |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability | 45,613 | 60,808 | 52,729 | 45,613 | 52,729 | 60,808 | (100,282) | 49,908 | (102,588)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings | (2,682) |  | 54,115 | (12,057) | 51,298 |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss |  | 0 | 0 | 0 | 0 |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases |  | 0 | 0 | 0 | 0 |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales |  | 0 | 0 | 0 | 0 |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements | 1,613 |  | (101,202) | 3,138 | 101,713 |  |  |  |
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held | (2,773) |  | 4,240 | (12,254) | 3,914 |  |  |  |
Derivative [Member] | Interest Rate Swap [Member] |  |  |  |  |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |  |  |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability | 46,939 | 61,374 |  | 46,939 |  | 61,374 |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings |  |  |  | (11,494) |  |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss |  |  |  | 0 |  |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases |  |  |  | 0 |  |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales |  |  |  | 0 |  |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements |  |  |  | 2,941 |  |  |  |  |
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held |  |  |  | (11,494) |  |  |  |  |
Derivative [Member] | Credit Derivative [Member] |  |  |  |  |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |  |  |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability | (1,326) | (566) |  | (1,326) |  | $ (566) |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings |  |  |  | (563) |  |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss |  |  |  | 0 |  |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases |  |  |  | 0 |  |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales |  |  |  | 0 |  |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements |  |  |  | 197 |  |  |  |  |
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held |  |  |  | (760) |  |  |  |  |
Credit Derivative [Member] | Derivative [Member] |  |  |  |  |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |  |  |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability |  |  | (9,006) |  | (9,006) |  | (15,349) |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings |  |  |  |  | 7,676 |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss |  |  |  |  | 0 |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases |  |  |  |  | 0 |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales |  |  |  |  | 0 |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements |  |  |  |  | (1,333) |  |  |  |
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held |  |  |  |  | 51 |  |  |  |
Credit Derivative [Member] | Derivative [Member] | Credit Derivative [Member] |  |  |  |  |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |  |  |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability | (1,326) |  | (9,006) | (1,326) | (9,006) |  |  | (1,018) | (14,496)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings | (217) |  | 6,624 |  |  |  |  |  |
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 Sales |  | 0 | 0 |  |  |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements | 91 |  | 1,134 |  |  |  |  |  |
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held | (308) |  | 191 |  |  |  |  |  |
Interest Rate Swap [Member] | Derivative [Member] |  |  |  |  |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |  |  |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability |  |  | 61,735 |  | 61,735 |  | $ (84,933) |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings |  |  |  |  | 43,622 |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss |  |  |  |  | 0 |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases |  |  |  |  | 0 |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales |  |  |  |  | 0 |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements |  |  |  |  | 103,046 |  |  |  |
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held |  |  |  |  | 3,863 |  |  |  |
Interest Rate Swap [Member] | Derivative [Member] | Interest Rate Swap [Member] |  |  |  |  |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |  |  |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability | 46,939 |  | 61,735 | $ 46,939 | $ 61,735 |  |  | $ 50,926 | $ (88,092)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings | (2,465) |  | 47,491 |  |  |  |  |  |
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 Sales |  | $ 0 | 0 |  |  |  |  |  |
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements | 1,522 |  | (102,336) |  |  |  |  |  |
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held | $ (2,465) |  | $ 4,049 |  |  |  |  |  |

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