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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 |  |  |  |

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- Definition The increase in basis points to the base interest rate in the event of a default on the credit agreement. + References No definition available. + Details
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- Definition The stated amount of the senior unsecured credit agreement which does not include the amount by which the credit facility can be increased. + References No definition available. + Details
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- Definition Date the debt instrument was issued, in CCYY-MM-DD format. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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- Definition Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(2)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb)(2) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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- Definition Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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