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Fair Value Measurements - Additional Information (Detail) - USD ($)
$ in Thousands
 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
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Equity in the earnings of liability companies or partnerships       $ 0 $ 0 $ 0 $ 0  
Equity Method Investments                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Equity in the earnings of liability companies or partnerships       $ 3,500 $ 1,300 $ (8,000) $ 1,400  
Variable Interest Entity, Not Primary Beneficiary                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Ownership interest exceeds respective investments       3.00%   3.00%    
7.75% Subordinated Notes due 2045                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Subordinated Notes due date 2045   2045     2045   2045
7.875% Subordinated Notes due 2047                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Subordinated Notes due date 2047 2047       2047   2047
Fair Value, Inputs, Level 1 | 7.75% Subordinated Notes due 2045                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Subordinated Notes due date           2045    
Fair Value, Inputs, Level 1 | 7.875% Subordinated Notes due 2047                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Subordinated Notes due date           2047