Real Estate Transactions - Additional Information (Detail) MMcf in Millions						3 Months Ended			9 Months Ended
	Sep. 21, 2018 USD ($) ft² Option MMcf	Aug. 23, 2018 USD ($) ft²	Jul. 17, 2018 USD ($) Property	Jan. 24, 2018 Bed	Jan. 19, 2018 USD ($) ft²	Sep. 30, 2018 USD ($) Bed	Jun. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Facility Bed	Sep. 30, 2017 USD ($)
Facility Activations Developments And Closures [Line Items]
Purchase price of real estate			$ 12,000,000
Percentage of building leased			100.00%
Purchase price, net tangible assets			$ 11,100,000
Purchase price, identifiable intangible assets			$ 1,700
Number of properties acquired | Property			12
Number of properties that may be marketed under portfolio acquisition			up to three
Business acquisition assumed in-place financing									$ 157,280,000
Operating Expense						$ 333,759,000		$ 316,865,000	973,449,000	$ 940,065,000
Proceeds from sale of corporate headquarters							$ 12,600,000
Asset impairments							$ 1,600,000	355,000	$ 1,580,000	614,000
Idled Correctional Facilities
Facility Activations Developments And Closures [Line Items]
Number of facility | Facility									8
Idled Non-Core Facilities
Facility Activations Developments And Closures [Line Items]
Number of beds at the facility | Bed						440			440
Number of facility | Facility									2
Net Carrying Value						$ 3,900,000			$ 3,900,000
Idle Facilities
Facility Activations Developments And Closures [Line Items]
Operating Expense						3,000,000		$ 3,000,000	9,700,000	$ 8,000,000
Kansas Department Of Corrections
Facility Activations Developments And Closures [Line Items]
Lease term				20 years
Number of beds at the facility | Bed				2,432
Construction of new facility expected commencement description				Construction of the new facility commenced in the first quarter of 2018 with a timeline for completion of approximately 24 months
Construction of new facility approximate completion term				24 months
Construction project capitalized amount						$ 31,800,000			$ 31,800,000
Capital Commerce Center
Facility Activations Developments And Closures [Line Items]
Area of building acquired | ft²					261,000
Purchase price of real estate					$ 44,700,000
Percentage of building leased					98.00%
Purchase price, net tangible assets					$ 40,600,000
Purchase price, identifiable intangible assets					$ 3,200,000
Social Security Administration
Facility Activations Developments And Closures [Line Items]
Area of building acquired | ft²		541,000
Purchase price of real estate		$ 242,000,000
Purchase price, net tangible assets		207,400,000
Purchase price, identifiable intangible assets		$ 38,900,000
Lease term		20 years
Lease term expiration date		Jan. 31, 2034
Business acquisition assumed in-place financing		$ 157,300,000
National Archives And Records Administration
Facility Activations Developments And Closures [Line Items]
Area of building acquired | ft²	217,000
Purchase price of real estate	$ 6,900,000
Percentage of building leased	100.00%
Purchase price, net tangible assets	$ 6,900,000
Purchase price, identifiable intangible assets	$ 700,000
Lease term expiration date	Jan. 31, 2023
Additional ten year renewal options | Option	2
Renewal of contract terms	10 years
Volume of storage space | MMcf	1.2
Percentage of storage space	90.00%
Florida | Capital Commerce Center
Facility Activations Developments And Closures [Line Items]
Percentage of building leased					87.00%