Debt - Additional Information (Detail) - USD ($)		1 Months Ended			9 Months Ended	12 Months Ended
	Jun. 21, 2018	Dec. 31, 2022	Feb. 28, 2022	Oct. 31, 2021	Sep. 30, 2023	Dec. 31, 2022
Fifth Amended and Restated Prudential Agreement [Member]
Credit and Loan Agreement [Line Items]
Amount of rate increase in case of default					2.00%
First Amended and Restated AIG Agreement [Member] | Series G Notes [Member]
Credit and Loan Agreement [Line Items]
Aggregate principal amount			$ 50,000,000
Senior unsecured note purchase agreement, maturity date			Sep. 12, 2029
Interest rate on agreement			3.52%
First Amended and Restated AIG Agreement [Member] | Series J Notes [Member]
Credit and Loan Agreement [Line Items]
Aggregate principal amount			$ 50,000,000
Senior unsecured note purchase agreement, maturity date			Nov. 25, 2030
Interest rate on agreement			3.43%
First Amended and Restated AIG Agreement [Member] | Series L Notes [Member]
Credit and Loan Agreement [Line Items]
Aggregate principal amount			$ 55,000,000
Senior unsecured note purchase agreement, maturity date			Feb. 22, 2032
Interest rate on agreement			3.45%
First Amended and Restated MassMutual Agreement [Member] | Series H Notes [Member]
Credit and Loan Agreement [Line Items]
Aggregate principal amount			$ 25,000,000
Senior unsecured note purchase agreement, maturity date			Sep. 12, 2029
Interest rate on agreement			3.52%
First Amended and Restated MassMutual Agreement [Member] | Series K Notes [Member]
Credit and Loan Agreement [Line Items]
Aggregate principal amount			$ 25,000,000
Senior unsecured note purchase agreement, maturity date			Nov. 25, 2030
Interest rate on agreement			3.43%
First Amended and Restated MassMutual Agreement [Member] | Series M Notes [Member]
Credit and Loan Agreement [Line Items]
Aggregate principal amount			$ 20,000,000
Senior unsecured note purchase agreement, maturity date			Feb. 22, 2032
Interest rate on agreement			3.45%
First Amended and Restated MassMutual Agreement [Member] | Series O Notes [Member]
Credit and Loan Agreement [Line Items]
Aggregate principal amount			$ 20,000,000
Senior unsecured note purchase agreement, maturity date			Jan. 20, 2033
Interest rate on agreement			3.65%
Sixth Amended And Restated Prudential Agreement [Member] | Series B Notes [Member]
Credit and Loan Agreement [Line Items]
Aggregate principal amount			$ 75,000,000
Senior unsecured note purchase agreement, maturity date			Jun. 02, 2023
Interest rate on agreement			5.35%
Sixth Amended And Restated Prudential Agreement [Member] | Series C Notes [Member]
Credit and Loan Agreement [Line Items]
Aggregate principal amount			$ 50,000,000
Senior unsecured note purchase agreement, maturity date			Feb. 25, 2025
Interest rate on agreement			4.75%
Sixth Amended And Restated Prudential Agreement [Member] | Series D Notes [Member]
Credit and Loan Agreement [Line Items]
Aggregate principal amount			$ 50,000,000
Senior unsecured note purchase agreement, maturity date			Jun. 21, 2028
Interest rate on agreement			5.47%
Sixth Amended And Restated Prudential Agreement [Member] | Series F Notes [Member]
Credit and Loan Agreement [Line Items]
Aggregate principal amount			$ 50,000,000
Senior unsecured note purchase agreement, maturity date			Sep. 12, 2029
Interest rate on agreement			3.52%
Sixth Amended And Restated Prudential Agreement [Member] | Series I Notes [Member]
Credit and Loan Agreement [Line Items]
Aggregate principal amount			$ 100,000,000
Senior unsecured note purchase agreement, maturity date			Nov. 25, 2030
Interest rate on agreement			3.43%
Sixth Amended And Restated Prudential Agreement [Member] | Series Q Notes [Member]
Credit and Loan Agreement [Line Items]
Aggregate principal amount			$ 80,000,000
Senior unsecured note purchase agreement, maturity date			Jan. 20, 2033
Interest rate on agreement			3.65%
New York Life Agreement [Member] | Series N Notes [Member]
Credit and Loan Agreement [Line Items]
Aggregate principal amount			$ 25,000,000
Senior unsecured note purchase agreement, maturity date			Feb. 22, 2032
Interest rate on agreement			3.45%
New York Life Agreement [Member] | Series P Notes [Member]
Credit and Loan Agreement [Line Items]
Aggregate principal amount			$ 25,000,000
Senior unsecured note purchase agreement, maturity date			Jan. 20, 2033
Interest rate on agreement			3.65%
Met Life Agreement [Member] | Series E Notes [Member]
Credit and Loan Agreement [Line Items]
Aggregate principal amount	$ 50,000,000
Senior unsecured note purchase agreement, maturity date	Jun. 21, 2028
Interest rate on agreement	5.47%
Revolving Credit Facility [Member]
Credit and Loan Agreement [Line Items]
Aggregate principal amount				$ 300,000,000
Line of credit facility, maturity date				Oct. 27, 2025
Line of credit facility, extension term					two six-month extensions (for a total of 12 months)
Senior unsecured note purchase agreement, maturity date					Oct. 27, 2025
Interest rate on agreement					6.70%
Revolving Credit Facility [Member] | SOFR [Member]
Credit and Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate						0.10%
Revolving Credit Facility [Member] | Maximum [Member]
Credit and Loan Agreement [Line Items]
Aggregate principal amount				$ 300,000,000
Annual commitment fee on undrawn funds		0.25%
Revolving Credit Facility [Member] | Maximum [Member] | Base Rate [Member]
Credit and Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate						1.90%
Revolving Credit Facility [Member] | Maximum [Member] | LIBOR [Member]
Credit and Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate						0.90%
Revolving Credit Facility [Member] | Minimum [Member]
Credit and Loan Agreement [Line Items]
Annual commitment fee on undrawn funds		0.15%
Revolving Credit Facility [Member] | Minimum [Member] | Base Rate [Member]
Credit and Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate						1.30%
Revolving Credit Facility [Member] | Minimum [Member] | LIBOR [Member]
Credit and Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate						0.30%