Concentration (Details)	9 Months Ended
Sep. 30, 2019 government property government_tenant state_government state	Sep. 30, 2018 government_tenant state_government
Concentration
Number of US government \| government	1
Number of state governments \| state_government	13	13
Number of other governments \| government_tenant	3	3
Annualized rental income, excluding properties classified as discontinued operations \| Tenant concentration \| U.S. Government, state governments and Other Four Government
Concentration
Concentration risk percentage	36.30%	62.50%
Annualized rental income, excluding properties classified as discontinued operations \| Tenant concentration \| U.S. Government
Concentration
Concentration risk percentage	25.80%	45.60%
Annualized rental income, excluding properties classified as discontinued operations \| Geographic Concentration Risk \| Virginia
Concentration
Concentration risk percentage	15.80%
Annualized rental income, excluding properties classified as discontinued operations \| Geographic Concentration Risk \| California
Concentration
Concentration risk percentage	11.60%
Annualized rental income, excluding properties classified as discontinued operations \| Geographic Concentration Risk \| Washington, D.C.
Concentration
Concentration risk percentage	9.80%
Annualized rental income, excluding properties classified as discontinued operations \| Geographic Concentration Risk \| Texas
Concentration
Concentration risk percentage	9.70%
Annualized rental income, excluding properties classified as discontinued operations \| Geographic Concentration Risk \| Maryland
Concentration
Concentration risk percentage	7.40%
Annualized rental income, excluding properties classified as discontinued operations \| Geographic Concentration Risk \| Metropolitan Washington D.C.
Concentration
Concentration risk percentage	24.60%
Continuing operations
Concentration
Number of wholly owned properties \| property	200
Number of states in which acquired properties located \| state	36

Number Of Governments

No definition available.

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- Definition

Represents the number of other governments that are tenants of the entity.

+ References

No definition available.

+ Details

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- Definition

Represents the number of state governments that are tenants of the entity.

+ References

No definition available.

+ Details

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- Definition

The number of states in which real estate owned by the entity is located as of the balance sheet date.

+ References

No definition available.

+ Details

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13531-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13537-108611

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

+ Details

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Period Type:	duration

- Definition

The number of real estate properties owned as of the balance sheet date.

+ References

No definition available.

+ Details

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