The amount by which the credit facility can be increased.

No definition available.

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Namespace Prefix:	gty_
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Balance Type:	credit
Period Type:	duration

- Definition

Credit and Loan Agreement [Line Items]

+ References

No definition available.

+ Details

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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

Line of credit facility agreement maturity date.

+ References

No definition available.

+ Details

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- Definition

Line of credit facility extended maturity date.

+ References

No definition available.

+ Details

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- Definition

Line of credit facility optional extended maturity date.

+ References

No definition available.

+ Details

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- Definition

The length of time the Revolving Facility may be extended.

+ 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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Namespace Prefix:	gty_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ Details

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Data Type:	xbrli:monetaryItemType
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Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Balance Type:	na
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- Definition

Date the debt instrument was issued, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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=6877327&loc=d3e13212-122682

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Period Type:	duration

- 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/presentationRef
-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=6877327&loc=d3e13212-122682

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Period Type:	duration

- 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://www.xbrl.org/2003/role/presentationRef
-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=6877327&loc=d3e13212-122682

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Date the credit facility first became available, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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=6877327&loc=d3e13212-122682

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Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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=6877327&loc=d3e13212-122682

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Balance Type:	credit
Period Type:	instant

- Definition

The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility.

+ References

No definition available.

+ Details

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- Definition

Including the current and noncurrent portions, aggregate carrying value as of the balance sheet date of loans payable (with maturities initially due after one year or beyond the operating cycle if longer).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
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-Publisher FASB
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-Paragraph 1
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16)(a)(2))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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Debt - Additional Information (Detail) - USD ($)			6 Months Ended	12 Months Ended
Debt - Additional Information (Detail) - USD ($)	Mar. 23, 2018	Jun. 02, 2015	Jun. 30, 2018	Dec. 31, 2017
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
Revolving facility optional extension period		1 year
Third Restated Prudential Note Purchase Agreement [Member]
Credit and Loan Agreement [Line Items]
Senior unsecured note, issuance date			Jun. 21, 2018
Amount of rate increase in case of default			2.00%
Third 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
Third 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
Third 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
Third 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
Third 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
Revolving Facility [Member]
Credit and Loan Agreement [Line Items]
Revolving facility under credit agreement		$ 175,000,000
Credit facility amount	$ 250,000,000
Revolving facility optional extension period			1 year
Credit facility agreement, optional extended maturity date			Mar. 31, 2023
Credit agreement maturity date	Jun. 30, 2018
Credit agreement extended maturity date	Mar. 23, 2022
Senior unsecured note purchase agreement, maturity date			Mar. 23, 2022	Mar. 23, 2022
Interest rate on agreement			3.53%	3.53%
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			$ 600,000,000
Credit agreement maturity date	Jun. 30, 2020
Credit agreement extended maturity date	Mar. 23, 2023
Senior unsecured note purchase agreement, maturity date			Mar. 23, 2023	Mar. 23, 2023
Interest rate on agreement			3.54%	3.54%
Term Loan [Member] \| Maximum [Member] \| Base Rate [Member]
Credit and Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate			1.25%
Term Loan [Member] \| Maximum [Member] \| LIBOR [Member]
Credit and Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate			2.25%
Term Loan [Member] \| Minimum [Member] \| Base Rate [Member]
Credit and Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate			0.45%
Term Loan [Member] \| Minimum [Member] \| LIBOR [Member]
Credit and Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate			1.45%