Debt - Additional Information (Detail) - USD ($)			6 Months Ended	12 Months Ended
	Mar. 23, 2018	Jun. 02, 2015	Jun. 30, 2020	Dec. 31, 2019	Sep. 12, 2019
Credit and Loan Agreement [Line Items]
Credit agreement initiation date		Jun. 02, 2015
Senior unsecured revolving credit agreement		$ 225,000,000
Term loan under credit agreement		50,000,000
Fourth Restated Prudential Note Purchase Agreement [Member]
Credit and Loan Agreement [Line Items]
Senior unsecured note, issuance date			Sep. 12, 2019
Amount of rate increase in case of default			2.00%
Fourth Restated Prudential Note Purchase Agreement [Member] | Series A Note [Member]
Credit and Loan Agreement [Line Items]
Senior unsecured note purchase agreement, maturity date			Feb. 25, 2021
Interest rate on agreement			6.00%
Senior unsecured note, aggregate amount issued			$ 100,000,000
Fourth Restated Prudential Note Purchase Agreement [Member] | Series B Notes [Member]
Credit and Loan Agreement [Line Items]
Senior unsecured note purchase agreement, maturity date			Jun. 02, 2023
Interest rate on agreement			5.35%
Senior unsecured note, aggregate amount issued			$ 75,000,000
Fourth Restated Prudential Note Purchase Agreement [Member] | Series C Notes [Member]
Credit and Loan Agreement [Line Items]
Senior unsecured note purchase agreement, maturity date			Feb. 25, 2025
Interest rate on agreement			4.75%
Senior unsecured note, aggregate amount issued			$ 50,000,000
Fourth Restated Prudential Note Purchase Agreement [Member] | Series D Notes [Member]
Credit and Loan Agreement [Line Items]
Senior unsecured note purchase agreement, maturity date			Jun. 21, 2028
Interest rate on agreement			5.47%
Senior unsecured note, aggregate amount issued			$ 50,000,000
Fourth Restated Prudential Note Purchase Agreement [Member] | Series E Notes [Member]
Credit and Loan Agreement [Line Items]
Senior unsecured note purchase agreement, maturity date			Jun. 21, 2028
Interest rate on agreement			5.47%
Senior unsecured note, aggregate amount issued			$ 50,000,000
Fourth Restated Prudential Note Purchase Agreement [Member] | Series F Notes [Member]
Credit and Loan Agreement [Line Items]
Senior unsecured note purchase agreement, maturity date			Sep. 12, 2029
Interest rate on agreement			3.52%
Senior unsecured note, aggregate amount issued			$ 50,000,000
Fourth Restated Prudential Note Purchase Agreement [Member] | Series G Notes [Member]
Credit and Loan Agreement [Line Items]
Senior unsecured note purchase agreement, maturity date			Sep. 12, 2029
Interest rate on agreement			3.52%
Senior unsecured note, aggregate amount issued			$ 50,000,000
Fourth Restated Prudential Note Purchase Agreement [Member] | Series H Notes [Member]
Credit and Loan Agreement [Line Items]
Senior unsecured note purchase agreement, maturity date			Sep. 12, 2029
Interest rate on agreement			3.52%
Senior unsecured note, aggregate amount issued			$ 25,000,000
Revolving Facility [Member]
Credit and Loan Agreement [Line Items]
Revolving facility under credit agreement	$ 175,000,000	$ 175,000,000			$ 250,000,000
Credit facility amount	$ 250,000,000		$ 600,000,000		300,000,000
Credit agreement maturity date	Jun. 30, 2018
Credit agreement extended maturity date	Mar. 23, 2022
Revolving facility optional extension period			1 year
Credit facility agreement, optional extended maturity date			Mar. 31, 2023
Senior unsecured note purchase agreement, maturity date			Mar. 31, 2022	Mar. 31, 2022
Interest rate on agreement			1.88%	1.88%
Revolving Facility [Member] | Maximum [Member]
Credit and Loan Agreement [Line Items]
Option to increase credit facility			$ 300,000,000
Annual commitment fee on undrawn funds			0.25%
Revolving Facility [Member] | Maximum [Member] | Base Rate [Member]
Credit and Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate			1.30%
Revolving Facility [Member] | Maximum [Member] | LIBOR [Member]
Credit and Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate			2.30%
Revolving Facility [Member] | Minimum [Member]
Credit and Loan Agreement [Line Items]
Annual commitment fee on undrawn funds			0.15%
Revolving Facility [Member] | Minimum [Member] | Base Rate [Member]
Credit and Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate			0.50%
Revolving Facility [Member] | Minimum [Member] | LIBOR [Member]
Credit and Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate			1.50%
Term Loan [Member]
Credit and Loan Agreement [Line Items]
Credit facility amount					$ 0
Credit agreement maturity date	Jun. 30, 2020
Credit agreement extended maturity date	Mar. 23, 2023