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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.