Rental Property - Acquisitions (Details) $ in Thousands		3 Months Ended			9 Months Ended
	Jan. 22, 2015 USD ($)	Sep. 30, 2015 USD ($) ft² building	Jun. 30, 2015 USD ($) ft² building	Mar. 31, 2015 USD ($) ft² building	Sep. 30, 2015 USD ($) ft² building
Acquired during the period
Area (in square feet)		52,100,000			52,100,000
2015 Q1 Acquisitions
Acquired during the period
Area (in square feet)				1,485,717
Number of buildings acquired | building				5
Purchase price | $				$ 97,083
Burlington, NJ
Acquired during the period
Area (in square feet)				503,490
Number of buildings acquired | building				1
Purchase price | $	$ 34,900			$ 34,883
Greenville, SC
Acquired during the period
Area (in square feet)				157,500
Number of buildings acquired | building				1
Purchase price | $				$ 4,800
North Haven, CT
Acquired during the period
Area (in square feet)				824,727
Number of buildings acquired | building				3
Purchase price | $				$ 57,400
2015 Q2 Acquisitions
Acquired during the period
Area (in square feet)			1,639,383
Number of buildings acquired | building			12
Purchase price | $			$ 83,808
Plymouth, MI
Acquired during the period
Area (in square feet)			125,214
Number of buildings acquired | building			1
Purchase price | $			$ 6,000
Oakwood, OH
Acquired during the period
Area (in square feet)			75,000
Number of buildings acquired | building			1
Purchase price | $			$ 4,398
Stoughton, MA
Acquired during the period
Area (in square feet)			250,213
Number of buildings acquired | building			2
Purchase price | $			$ 10,675
Oklahoma City, OK
Acquired during the period
Area (in square feet)			223,340
Number of buildings acquired | building			1
Purchase price | $			$ 12,135
Clinton, TN
Acquired during the period
Area (in square feet)			166,000
Number of buildings acquired | building			1
Purchase price | $			$ 5,000
Knoxville, TN
Acquired during the period
Area (in square feet)			108,400
Number of buildings acquired | building			1
Purchase price | $			$ 4,750
Fairborn, OH
Acquired during the period
Area (in square feet)			258,680
Number of buildings acquired | building			1
Purchase price | $			$ 9,100
El Paso, TX
Acquired during the period
Area (in square feet)			126,456
Number of buildings acquired | building			1
Purchase price | $			$ 9,700
Phoenix, AZ
Acquired during the period
Area (in square feet)			102,747
Number of buildings acquired | building			1
Purchase price | $			$ 9,500
Charlotte, NC
Acquired during the period
Area (in square feet)			123,333
Number of buildings acquired | building			1
Purchase price | $			$ 7,500
Machesney Park, IL
Acquired during the period
Area (in square feet)			80,000
Number of buildings acquired | building			1
Purchase price | $			$ 5,050
2015 Q3 Acquisitions
Acquired during the period
Area (in square feet)		2,509,084			2,509,084
Number of buildings acquired | building		18
Purchase price | $		$ 108,286
Macedonia, OH
Acquired during the period
Area (in square feet)		201,519			201,519
Number of buildings acquired | building		1
Purchase price | $		$ 12,192
Novi, MI
Acquired during the period
Area (in square feet)		125,060			125,060
Number of buildings acquired | building		1
Purchase price | $		$ 8,716
Grand Junction, CO
Acquired during the period
Area (in square feet)		82,800			82,800
Number of buildings acquired | building		1
Purchase price | $		$ 5,254
Tulsa, OK
Acquired during the period
Area (in square feet)		175,000			175,000
Number of buildings acquired | building		1
Purchase price | $		$ 13,000
Chattanooga, TN
Acquired during the period
Area (in square feet)		646,200			646,200
Number of buildings acquired | building		3
Purchase price | $		$ 21,160
Libertyville, IL
Acquired during the period
Area (in square feet)		287,102			287,102
Number of buildings acquired | building		2
Purchase price | $		$ 11,121
Greer, SC
Acquired during the period
Area (in square feet)		290,000			290,000
Number of buildings acquired | building		4
Purchase price | $		$ 9,025
Piedmont, SC
Acquired during the period
Area (in square feet)		400,000			400,000
Number of buildings acquired | building		3
Purchase price | $		$ 12,000
Belvidere, IL
Acquired during the period
Area (in square feet)		100,000			100,000
Number of buildings acquired | building		1
Purchase price | $		$ 5,938
Conyers, GA
Acquired during the period
Area (in square feet)		201,403			201,403
Number of buildings acquired | building		1
Purchase price | $		$ 9,880
2015 Q3 YTD Acquisitions
Acquired during the period
Area (in square feet)		5,634,184			5,634,184
Number of buildings acquired | building					35
Purchase price | $					$ 289,177