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Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Detail) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
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Standard & Poor's, CC Rating [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | $ 2,104,499
Standard & Poor's, CC Rating [Member] | Ambac Assurance Corporation [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | $ 1,349,832 | 2,088,899
Standard & Poor's, BBB- Rating [Member] | National Public Finance Guarantee Corporation [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 13,839 | 15,600
Standard & Poor's, CCC Rating [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 1,363,671 |
Municipal Bonds [Member] | Standard & Poor's, CC Rating [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 848,841
Municipal Bonds [Member] | Standard & Poor's, CC Rating [Member] | Ambac Assurance Corporation [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 166,770 | 833,241
Municipal Bonds [Member] | Standard & Poor's, BBB- Rating [Member] | National Public Finance Guarantee Corporation [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 13,839 | 15,600
Municipal Bonds [Member] | Standard & Poor's, CCC Rating [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 180,609 |
Corporate Debt Securities [Member] | Standard & Poor's, CC Rating [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 656,473
Corporate Debt Securities [Member] | Standard & Poor's, CC Rating [Member] | Ambac Assurance Corporation [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 583,550 | 656,473
Corporate Debt Securities [Member] | Standard & Poor's, BBB- Rating [Member] | National Public Finance Guarantee Corporation [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 0 | 0
Corporate Debt Securities [Member] | Standard & Poor's, CCC Rating [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 583,550 |
Mortgage and Asset-Backed Securities [Member] | Standard & Poor's, CC Rating [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 599,185
Mortgage and Asset-Backed Securities [Member] | Standard & Poor's, CC Rating [Member] | Ambac Assurance Corporation [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 599,512 | $ 599,185
Mortgage and Asset-Backed Securities [Member] | Standard & Poor's, CCC Rating [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | $ 599,512 |

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