Real Estate Properties and Accumulated Depreciation - Schedule of Real Estate Properties (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Initial Costs
Land	$ 92.4
Building and Improvements	312.6
Equipment	103.3
Cost Capitalized Subsequent to Acquisition
Land	0.6
Building and Improvements	592.7
Equipment	494.9
Gross Carrying Amount
Land	93.0	$ 89.7
Building and Improvements	905.3	812.6
Equipment	598.2	349.1
Accumulated Depreciation	435.6	327.0	$ 236.7	$ 176.7
Aggregate cost of the total properties for federal income tax purposes	2,419.1
West Seventh St., Cincinnati, OH (7th Street)
Initial Costs
Land	0.9
Building and Improvements	42.2
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	68.4
Equipment	19.6
Gross Carrying Amount
Land	0.9	0.9
Building and Improvements	110.6	110.6
Equipment	19.6	12.7
Accumulated Depreciation	75.1
Parkway Dr., Mason, OH (Mason)
Initial Costs
Land	0.0
Building and Improvements	0.0
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	20.2
Equipment	1.0
Gross Carrying Amount
Land	0.0	0.0
Building and Improvements	20.2	20.2
Equipment	1.0	0.9
Accumulated Depreciation	11.8
Industrial Rd., Florence, KY (Florence)
Initial Costs
Land	2.2
Building and Improvements	7.7
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	33.8
Equipment	3.3
Gross Carrying Amount
Land	2.2	2.2
Building and Improvements	41.5	41.4
Equipment	3.3	3.0
Accumulated Depreciation	21.8
Goldcoast Dr., Cincinnati, OH (Goldcoast)
Initial Costs
Land	0.6
Building and Improvements	0.0
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	6.7
Equipment	0.1
Gross Carrying Amount
Land	0.6	0.6
Building and Improvements	6.7	6.7
Equipment	0.1	0.1
Accumulated Depreciation	2.6
Knightsbridge Dr., Hamilton, OH (Hamilton)
Initial Costs
Land	0.0
Building and Improvements	9.5
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	39.7
Equipment	4.4
Gross Carrying Amount
Land	0.0	0.0
Building and Improvements	49.2	49.2
Equipment	4.4	3.7
Accumulated Depreciation	22.7
E. Monroe St., South Bend, IN (Monroe St.)
Initial Costs
Land	0.0
Building and Improvements	0.0
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	2.5
Equipment	0.3
Gross Carrying Amount
Land	0.0	0.0
Building and Improvements	2.5	2.5
Equipment	0.3	0.1
Accumulated Depreciation	1.3
Springer St., Lombard, IL (Lombard)
Initial Costs
Land	0.7
Building and Improvements	3.2
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	1.5
Equipment	7.6
Gross Carrying Amount
Land	0.7	0.7
Building and Improvements	4.7	4.7
Equipment	7.6	5.7
Accumulated Depreciation	2.9
Crescent Circle, South Bend, IN (Blackthorn)
Initial Costs
Land	0.0
Building and Improvements	1.1
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	2.2
Equipment	0.4
Gross Carrying Amount
Land	0.0	0.0
Building and Improvements	3.3	3.3
Equipment	0.4	0.1
Accumulated Depreciation	1.5
Kingsview Dr., Lebanon, OH (Lebanon)
Initial Costs
Land	4.0
Building and Improvements	12.3
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	65.0
Equipment	7.6
Gross Carrying Amount
Land	4.0	4.0
Building and Improvements	77.3	77.0
Equipment	7.6	5.5
Accumulated Depreciation	26.5
McAuley Place, Blue Ash, OH (Blue Ash)
Initial Costs
Land	0.0
Building and Improvements	2.6
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	(2.0)
Equipment	0.1
Gross Carrying Amount
Land	0.0	0.0
Building and Improvements	0.6	0.6
Equipment	0.1	0.1
Accumulated Depreciation	0.3
Westway Park Blvd., Houston, TX (Houston West 1)
Initial Costs
Land	1.4
Building and Improvements	21.4
Equipment	0.1
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	63.4
Equipment	46.3
Gross Carrying Amount
Land	1.4	1.4
Building and Improvements	84.8	84.4
Equipment	46.4	43.8
Accumulated Depreciation	51.7
Westway Park Blvd., Houston, TX (Houston West 2)
Initial Costs
Land	2.0
Building and Improvements	0.0
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	22.6
Equipment	47.1
Gross Carrying Amount
Land	2.0	2.0
Building and Improvements	22.6	22.5
Equipment	47.1	45.1
Accumulated Depreciation	15.3
Westway Park Blvd., Houston, TX (Houston West 3)
Initial Costs
Land	18.3
Building and Improvements	0.0
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.1
Building and Improvements	4.0
Equipment	0.8
Gross Carrying Amount
Land	18.4	18.4
Building and Improvements	4.0	0.0
Equipment	0.8	0.0
Accumulated Depreciation	0.3
Southwest Fwy., Houston, TX (Galleria)
Initial Costs
Land	0.0
Building and Improvements	56.0
Equipment	2.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	12.6
Equipment	14.0
Gross Carrying Amount
Land	0.0	0.0
Building and Improvements	68.6	68.6
Equipment	16.0	15.0
Accumulated Depreciation	36.8
E. Ben White Blvd., Austin, TX (Austin 1)
Initial Costs
Land	0.0
Building and Improvements	11.9
Equipment	0.2
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	1.7
Equipment	0.8
Gross Carrying Amount
Land	0.0	0.0
Building and Improvements	13.6	22.5
Equipment	1.0	1.2
Accumulated Depreciation	10.1
S. State Highway 121 Business, Lewisville, TX (Lewisville)
Initial Costs
Land	0.0
Building and Improvements	46.2
Equipment	2.2
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	30.4
Equipment	22.7
Gross Carrying Amount
Land	0.0	0.0
Building and Improvements	76.6	76.7
Equipment	24.9	22.8
Accumulated Depreciation	45.3
Marsh Lane, Carrollton, TX (Marsh Ln)
Initial Costs
Land	0.0
Building and Improvements	0.0
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	0.1
Equipment	0.6
Gross Carrying Amount
Land	0.0	0.0
Building and Improvements	0.1	0.1
Equipment	0.6	0.5
Accumulated Depreciation	0.4
Midway Rd., Carrollton, TX (Midway)
Initial Costs
Land	0.0
Building and Improvements	1.8
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	0.2
Equipment	0.4
Gross Carrying Amount
Land	0.0	0.0
Building and Improvements	2.0	2.0
Equipment	0.4	0.4
Accumulated Depreciation	2.2
W. Frankford Rd., Carrollton, TX (Carrollton)
Initial Costs
Land	16.1
Building and Improvements	0.0
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	52.7
Equipment	116.5
Gross Carrying Amount
Land	16.1	16.1
Building and Improvements	52.7	51.6
Equipment	116.5	85.3
Accumulated Depreciation	34.3
Bryan St., Dallas, TX (Bryan St)
Initial Costs
Land	0.0
Building and Improvements	0.1
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	(0.1)
Equipment	0.0
Gross Carrying Amount
Land	0.0	0.0
Building and Improvements	0.0	0.1
Equipment	0.0	0.2
Accumulated Depreciation	0.0
North Freeway, Houston, TX (Greenspoint)
Initial Costs
Land	0.0
Building and Improvements	0.0
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	0.0
Equipment	0.0
Gross Carrying Amount
Land	0.0	0.0
Building and Improvements	0.0	1.3
Equipment	0.0	0.0
Accumulated Depreciation	0.0
South Ellis Street, Chandler, AZ (Phoenix 1)
Initial Costs
Land	14.8
Building and Improvements	0.0
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	56.7
Equipment	39.8
Gross Carrying Amount
Land	14.8	14.8
Building and Improvements	56.7	56.4
Equipment	39.8	43.9
Accumulated Depreciation	18.5
South Ellis Street, Chandler, AZ (Phoenix 2)
Initial Costs
Land	0.0
Building and Improvements	0.0
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	16.0
Equipment	39.5
Gross Carrying Amount
Land	0.0	0.0
Building and Improvements	16.0	13.2
Equipment	39.5	21.8
Accumulated Depreciation	5.3
Westover Hills Blvd., San Antonio, TX (San Antonio 1)
Initial Costs
Land	4.6
Building and Improvements	3.0
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	29.1
Equipment	33.0
Gross Carrying Amount
Land	4.6	4.6
Building and Improvements	32.1	32.1
Equipment	33.0	32.4
Accumulated Depreciation	15.9
Westover Hills Blvd., San Antonio, TX (San Antonio 2)
Initial Costs
Land	6.7
Building and Improvements	0.0
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.3
Building and Improvements	0.0
Equipment	0.1
Gross Carrying Amount
Land	7.0	7.0
Building and Improvements	0.0	0.0
Equipment	0.1	0.0
Accumulated Depreciation	0.0
Metropolis Dr., Austin, TX (Austin 2)
Initial Costs
Land	2.0
Building and Improvements	0.0
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	23.2
Equipment	5.7
Gross Carrying Amount
Land	2.0	2.0
Building and Improvements	23.2	23.2
Equipment	5.7	4.0
Accumulated Depreciation	9.6
Myer Conners Rd (Wappinger Falls)
Initial Costs
Land	0.0
Building and Improvements	9.9
Equipment	13.3
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	1.4
Equipment	1.1
Gross Carrying Amount
Land	0.0	0.0
Building and Improvements	11.3	0.0
Equipment	14.4	0.0
Accumulated Depreciation	1.3
Madison Road (Totowa)
Initial Costs
Land	0.0
Building and Improvements	28.3
Equipment	45.6
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	0.0
Equipment	3.2
Gross Carrying Amount
Land	0.0	0.0
Building and Improvements	28.3	0.0
Equipment	48.8	0.0
Accumulated Depreciation	3.8
Commerce Road (Totowa)
Initial Costs
Land	0.0
Building and Improvements	4.1
Equipment	0.8
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	0.0
Equipment	0.2
Gross Carrying Amount
Land	0.0	0.0
Building and Improvements	4.1	0.0
Equipment	1.0	0.0
Accumulated Depreciation	0.2
Norden Place (Norwalk)
Initial Costs
Land	0.0
Building and Improvements	18.3
Equipment	25.3
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	0.0
Equipment	0.1
Gross Carrying Amount
Land	0.0	0.0
Building and Improvements	18.3	0.0
Equipment	25.4	0.0
Accumulated Depreciation	1.6
Riverbend Drive South (Stamford)
Initial Costs
Land	0.0
Building and Improvements	4.3
Equipment	13.2
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	0.0
Equipment	0.0
Gross Carrying Amount
Land	0.0	0.0
Building and Improvements	4.3	0.0
Equipment	13.2	0.0
Accumulated Depreciation	1.2
Omega Drive (Stamford)
Initial Costs
Land	0.0
Building and Improvements	3.2
Equipment	0.6
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	0.0
Equipment	0.9
Gross Carrying Amount
Land	0.0	0.0
Building and Improvements	3.2	0.0
Equipment	1.5	0.0
Accumulated Depreciation	0.2
Kestral Way (London)
Initial Costs
Land	0.0
Building and Improvements	16.5
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	14.7
Equipment	0.8
Gross Carrying Amount
Land	0.0	0.0
Building and Improvements	31.2	32.7
Equipment	0.8	0.7
Accumulated Depreciation	6.0
Jurong East (Singapore)
Initial Costs
Land	0.0
Building and Improvements	9.0
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	(0.6)
Equipment	0.1
Gross Carrying Amount
Land	0.0	0.0
Building and Improvements	8.4	9.0
Equipment	0.1	0.1
Accumulated Depreciation	3.6
Ridgetop Circle, Sterling, VA (Northern Virginia)
Initial Costs
Land	6.9
Building and Improvements	0.0
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.1
Building and Improvements	19.2
Equipment	45.2
Gross Carrying Amount
Land	7.0	7.0
Building and Improvements	19.2	0.0
Equipment	45.2	0.0
Accumulated Depreciation	5.0
Metropolis Dr., Austin, TX (Austin 3)
Initial Costs
Land	7.9
Building and Improvements	0.0
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.1
Building and Improvements	0.0
Equipment	0.1
Gross Carrying Amount
Land	8.0	8.0
Building and Improvements	0.0	0.0
Equipment	0.1	0.0
Accumulated Depreciation	0.0
Metropolis Dr., Austin, TX (Austin 4)
Initial Costs
Land	3.3
Building and Improvements	0.0
Equipment	0.0
Cost Capitalized Subsequent to Acquisition
Land	0.0
Building and Improvements	7.4
Equipment	31.5
Gross Carrying Amount
Land	3.3	0.0
Building and Improvements	7.4	0.0
Equipment	31.5	$ 0.0
Accumulated Depreciation	$ 0.5