The amount by which the credit facility can be increased.

No definition available.

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Namespace Prefix:	gty_
Data Type:	xbrli:monetaryItemType
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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Balance Type:	na
Period Type:	duration

- Definition

Line of credit facility optional extended maturity date.

+ References

No definition available.

+ Details

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Balance Type:	na
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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
Balance Type:	credit
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
Period Type:	instant

- 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

+ Details

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Data Type:	xbrli:dateItemType
Balance Type:	na
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

+ Details

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Data Type:	xbrli:dateItemType
Balance Type:	na
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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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Date the credit facility terminates, 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

+ Details

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

- 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

+ Details

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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

+ Details

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Data Type:	xbrli:monetaryItemType
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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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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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-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
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-Publisher FASB
-Name Accounting Standards Codification
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-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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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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- Details

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- Details

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- Details

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Debt - Additional Information (Detail) - USD ($)		12 Months Ended
Debt - Additional Information (Detail) - USD ($)	Jun. 02, 2015	Dec. 31, 2017	Dec. 31, 2016
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
Second Restated Prudential Note Purchase Agreement [Member]
Credit And Loan Agreement [Line Items]
Senior unsecured note, issuance date		Feb. 21, 2017
Amount of rate increase in case of default		2.00%
Second 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
Second 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
Second 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
Minimum [Member]
Credit And Loan Agreement [Line Items]
Annual commitment fee on undrawn funds		0.25%
Maximum [Member]
Credit And Loan Agreement [Line Items]
Annual commitment fee on undrawn funds		0.30%
Revolving Facility [Member]
Credit And Loan Agreement [Line Items]
Revolving facility under credit agreement	175,000,000
Option to increase credit facility	75,000,000
Credit facility amount	$ 250,000,000
Revolving facility optional extension period		1 year
Credit facility agreement, maturity date		Jun. 02, 2018
Credit facility agreement, optional extended maturity date		Jun. 02, 2019
Senior unsecured note purchase agreement, maturity date		Jun. 02, 2018	Jun. 02, 2018
Interest rate on agreement		3.72%	3.33%
Revolving Facility [Member] \| Minimum [Member] \| Base Rate [Member]
Credit And Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate		0.95%
Revolving Facility [Member] \| Minimum [Member] \| LIBOR [Member]
Credit And Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate		1.95%
Revolving Facility [Member] \| Maximum [Member] \| Base Rate [Member]
Credit And Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate		2.25%
Revolving Facility [Member] \| Maximum [Member] \| LIBOR [Member]
Credit And Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate		3.25%
Term Loan [Member]
Credit And Loan Agreement [Line Items]
Senior unsecured note purchase agreement, maturity date		Jun. 02, 2020	Jun. 02, 2020
Interest rate on agreement		3.85%	3.28%
Term Loan [Member] \| Minimum [Member] \| Base Rate [Member]
Credit And Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate		0.90%
Term Loan [Member] \| Minimum [Member] \| LIBOR [Member]
Credit And Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate		1.90%
Term Loan [Member] \| Maximum [Member] \| Base Rate [Member]
Credit And Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate		2.20%
Term Loan [Member] \| Maximum [Member] \| LIBOR [Member]
Credit And Loan Agreement [Line Items]
Credit agreement margin on borrowing base rate		3.20%