Fair Value Measurements - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets, Transfers between level 1 and level 2	$ 0	$ 0	$ 0	$ 0
Fair value liability, Transfers between level 1 and level 2	0	0	0	0
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	$ 1,300,000	$ 1,600,000	$ 1,400,000	$ 5,200,000
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
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
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