Rental Property - Acquisitions (Details) $ in Thousands		3 Months Ended		6 Months Ended
	May 31, 2017 USD ($)	Jun. 30, 2017 USD ($) ft² building	Mar. 31, 2017 USD ($) ft² building	Jun. 30, 2017 USD ($) ft² building
Business Acquisition [Line Items]
Area (in square feet)		67,700,000		67,700,000
Acquisitions 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)		6,908,416		6,908,416
Number of Buildings Acquired | building				32
Business Combination, Consideration Transferred | $				$ 385,393
AcquisitionsQ12017 [Member]
Business Acquisition [Line Items]
Area (in square feet)			2,334,622
Number of Buildings Acquired | building			11
Business Combination, Consideration Transferred | $			$ 99,787
Jacksonville, FL, Q1 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)			1,025,720
Number of Buildings Acquired | building			4
Business Combination, Consideration Transferred | $			$ 34,264
Sparks, NV, Q1 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)			174,763
Number of Buildings Acquired | building			1
Business Combination, Consideration Transferred | $			$ 8,380
Salisbury, NC, Q1 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)			288,000
Number of Buildings Acquired | building			1
Business Combination, Consideration Transferred | $			$ 8,250
Franklin Township, NJ, Q1 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)			183,000
Number of Buildings Acquired | building			1
Business Combination, Consideration Transferred | $			$ 12,800
Milford, CT, Q1 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)			200,000
Number of Buildings Acquired | building			1
Business Combination, Consideration Transferred | $			$ 12,762
Bedford Heights, OH, Q1 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)			173,034
Number of Buildings Acquired | building			1
Business Combination, Consideration Transferred | $			$ 7,622
Redford, MI, Q1 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)			135,728
Number of Buildings Acquired | building			1
Business Combination, Consideration Transferred | $			$ 7,769
Warren, MI, Q1 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)			154,377
Number of Buildings Acquired | building			1
Business Combination, Consideration Transferred | $			$ 7,940
AcquisitionsQ22017 [Member]
Business Acquisition [Line Items]
Area (in square feet)		4,573,794		4,573,794
Number of Buildings Acquired | building		21
Business Combination, Consideration Transferred | $		$ 285,606
Waukegan, IL, Q2 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)		261,075		261,075
Number of Buildings Acquired | building		2
Business Combination, Consideration Transferred | $		$ 13,850
Gaffney, SC, Q2 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)		226,968		226,968
Number of Buildings Acquired | building		1
Business Combination, Consideration Transferred | $		$ 7,200
Dayton, OH, Q2 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)		569,966		569,966
Number of Buildings Acquired | building		1
Business Combination, Consideration Transferred | $		$ 29,750
Belvidere, IL, Q2 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)		336,204		336,204
Number of Buildings Acquired | building		1
Business Combination, Consideration Transferred | $		$ 22,867
San Diego, CA, Q2 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)		205,440		205,440
Number of Buildings Acquired | building		1
Business Combination, Consideration Transferred | $	$ 19,362	$ 19,362
Edwardsville, KS, Q2 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)		270,869		270,869
Number of Buildings Acquired | building		1
Business Combination, Consideration Transferred | $		$ 16,270
Pedricktown, NJ, Q2 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)		245,749		245,749
Number of Buildings Acquired | building		1
Business Combination, Consideration Transferred | $		$ 16,000
Walton, KY, Q2 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)		224,921		224,921
Number of Buildings Acquired | building		1
Business Combination, Consideration Transferred | $		$ 11,450
Rock HIll, SC, Q2 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)		275,000		275,000
Number of Buildings Acquired | building		1
Business Combination, Consideration Transferred | $		$ 6,675
Laredo, TX, Q2 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)		206,810		206,810
Number of Buildings Acquired | building		1
Business Combination, Consideration Transferred | $		$ 13,500
Clinton, PA, Q2 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)		297,200		297,200
Number of Buildings Acquired | building		1
Business Combination, Consideration Transferred | $		$ 23,650
Rockwall, TX, Q2 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)		389,546		389,546
Number of Buildings Acquired | building		1
Business Combination, Consideration Transferred | $		$ 28,600
Batavia, IL, Q2 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)		102,500		102,500
Number of Buildings Acquired | building		1
Business Combination, Consideration Transferred | $		$ 5,900
Romulus, MI, Q2 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)		303,760		303,760
Number of Buildings Acquired | building		1
Business Combination, Consideration Transferred | $		$ 19,351
Wallingford, CT, Q2 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)		105,000		105,000
Number of Buildings Acquired | building		1
Business Combination, Consideration Transferred | $		$ 8,200
Houston, TX, Q2 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)		232,800		232,800
Number of Buildings Acquired | building		3
Business Combination, Consideration Transferred | $		$ 25,000
Lebanon, PA, Q2 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)		211,358		211,358
Number of Buildings Acquired | building		1
Business Combination, Consideration Transferred | $		$ 7,950
Maple Grove, MN, Q2 2017 [Member]
Business Acquisition [Line Items]
Area (in square feet)		108,628		108,628
Number of Buildings Acquired | building		1
Business Combination, Consideration Transferred | $		$ 10,031