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Debt - Additional Information (Detail) - USD ($) |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | ---
Apr. 25, 2018 | Mar. 23, 2017 | Aug. 31, 2020 | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Dec. 31, 2019 | Mar. 30, 2017
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt |  |  |  |  | $ 126,253,000 |  | $ 126,253,000 |  | $ 296,640,000 |
Interest expense |  |  |  |  | 3,620,000 | $ 5,023,000 | 13,197,000 | $ 15,088,000 |  |
Redemption of aggregate principal amount |  |  |  |  |  |  | 100,000,000 | 0 |  |
Loss on extinguishment of debt |  |  |  |  | $ (3,060,000) | 0 | $ (3,060,000) | 0 |  |
Debt instrument, maturity date |  | Apr. 15, 2047 |  |  |  |  |  |  |  |
Promissory notes |  | $ 230,000,000 |  |  |  |  |  |  |  |
Promissory note settled |  |  |  |  |  |  | 2020-08 |  |  |
7.75% Subordinated Notes due 2045 |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  | 7.75% | 7.75% |  | 7.75% |  | 7.75% |
Debt | [1] |  |  |  | $ 0 |  | $ 0 |  | $ 96,864,000 |
Interest expense |  |  |  |  | $ 1,000,000.0 | 2,000,000.0 | $ 4,900,000 | 5,900,000 |  |
Subordinated Notes due date |  | 2045 |  | 2045 |  |  | 2045 |  | 2045 |
Redemption of aggregate principal amount |  |  |  |  |  |  | $ 100,000,000 |  |  |
Redemption date |  |  |  |  |  |  | Aug. 15, 2020 |  |  |
Loss on extinguishment of debt |  |  |  |  |  |  | $ 3,100 |  |  |
7.875% Subordinated Notes due 2047 |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  | 7.875% |  | 7.875% |  | 7.875% |
Debt | [2] |  |  |  | $ 126,253,000 |  | $ 126,253,000 |  | $ 126,147,000 |
Interest expense |  |  |  |  | 2,600,000 | 2,600,000 | $ 7,800,000 | 7,800,000 |  |
Subordinated Notes due date |  | 2047 | 2047 |  |  |  | 2047 |  | 2047 |
Debt instrument, face amount |  |  | $ 120,000.0 |  | 130,000.0 |  | $ 130,000.0 |  |  |
Number of days granted to underwriters option to purchase |  |  | 30 days |  |  |  |  |  |  |
Additional aggregate principal amount purchased pursuant to over-allotment option granted to the underwriters |  |  | $ 18,000 |  |  |  |  |  |  |
Debt instrument, interest rate terms |  |  |  |  |  |  | payable quarterly in arrears on January 15, April 15, July 15, and October 15 of each year |  |  |
Debt instrument, maturity date |  |  |  |  |  |  | Apr. 15, 2047 |  |  |
Debt instrument, redemption description |  |  |  |  |  |  | The Company has the right to redeem the 2047 Notes in $25 increments, in whole or in part, on and after April 15, 2022, or on any interest payment date thereafter, at a redemption price equal to 100% of the principal amount of the 2047 Notes being redeemed plus accrued and unpaid interest to, but not including, the date of redemption. |  |  |
Debt instrument, redemption price, percentage of principal amount redeemed |  |  |  |  |  |  | 100.00% |  |  |
Deferred issuance costs |  |  |  |  | $ 4,200,000 |  | $ 4,200,000 |  |  |
7.875% Subordinated Notes due 2047 | Over-Allotment Option |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt instrument, face amount |  |  |  |  |  |  |  |  |  | $ 10,000,000
Margin borrowing facilities |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  | 0.80% |  | 0.80% |  | 1.90% |
Collateral deposited to support borrowing |  |  |  |  | $ 0 |  | $ 0 |  | $ 88,200,000 |
Debt Securities, Available-for-sale, Restriction Type [Extensible List] |  |  |  |  | us-gaap:CollateralPledgedMember |  | us-gaap:CollateralPledgedMember |  |  |
Debt | [3] |  |  |  | $ 0 |  | $ 0 |  | $ 73,629,000 |
Interest expense |  |  |  |  | $ 100,000 | $ 500,000 | $ 500,000 | $ 1,400,000 |  |
[1] | As of December 31, 2019, the carrying value and fair value of the 7.75% Subordinated Notes due 2045 are net of unamortized debt issuance cost of $3.1 million. In August 2020, the Company redeemed all of its outstanding 7.75% subordinated notes due 2045 and unamortized debt issuance cost of $3.1 million was written off and included in the consolidated statements of operations as loss on the extinguishment of debt.
[2] | As of September 30, 2020 and December 31, 2019, the carrying value and fair value of the 7.875% Subordinated Notes due 2047 are net of unamortized debt issuance cost of $3.7 million and $3.9 million, respectively.
[3] | The Margin Borrowing Facility was fully paid down in August 2020.

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