Schedule III - Real Estate and Accumulated Depreciation - Property Summary (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) store	Dec. 31, 2016 USD ($)
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	847
Debt	$ 2,832,923
Land Intial Cost	1,749,559
Building and Improvements Intial Cost	5,897,098
Adjustments and Costs Subsequent to Acquisition	545,834
Gross carrying amount - Land	1,745,161
Gross carrying amount - Building and improvements	6,447,330
Gross carrying amount - Total	8,192,491
Accumulated Depreciation	1,060,060	$ 900,861
AL
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	5,629
AZ
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	16,873
CA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	194,614
CO
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	14,611
CT
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	6,657
FL
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	97,830
GA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	37,953
HI
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	17,887
IL
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	28,278
IN
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	7,870
KS
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	916
KY
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	7,675
LA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	3,615
MA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	63,898
MD
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	53,154
MI
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	2,744
MN
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	523
MO
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	6,143
MS
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	1,342
NC
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	5,391
NH
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	2,074
NJ
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	97,042
NM
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	5,103
NV
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	5,834
NY
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	44,859
OH
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	9,769
OR
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated Depreciation	$ 5,261
Self Storage Facility | AL
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	8
Debt	$ 28,937
Land Intial Cost	7,690
Building and Improvements Intial Cost	42,770
Adjustments and Costs Subsequent to Acquisition	2,749
Gross carrying amount - Land	7,691
Gross carrying amount - Building and improvements	45,518
Gross carrying amount - Total	$ 53,209
Self Storage Facility | AZ
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	22
Debt	$ 39,996
Land Intial Cost	24,250
Building and Improvements Intial Cost	109,844
Adjustments and Costs Subsequent to Acquisition	7,614
Gross carrying amount - Land	24,248
Gross carrying amount - Building and improvements	117,460
Gross carrying amount - Total	$ 141,708
Self Storage Facility | CA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	145
Debt	$ 669,690
Land Intial Cost	449,865
Building and Improvements Intial Cost	1,050,646
Adjustments and Costs Subsequent to Acquisition	82,601
Gross carrying amount - Land	450,072
Gross carrying amount - Building and improvements	1,133,040
Gross carrying amount - Total	$ 1,583,112
Self Storage Facility | CO
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	13
Debt	$ 28,411
Land Intial Cost	9,785
Building and Improvements Intial Cost	45,004
Adjustments and Costs Subsequent to Acquisition	12,959
Gross carrying amount - Land	9,972
Gross carrying amount - Building and improvements	57,776
Gross carrying amount - Total	$ 67,748
Self Storage Facility | CT
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	7
Debt	$ 14,364
Land Intial Cost	9,875
Building and Improvements Intial Cost	50,966
Adjustments and Costs Subsequent to Acquisition	3,351
Gross carrying amount - Land	9,874
Gross carrying amount - Building and improvements	54,318
Gross carrying amount - Total	$ 64,192
Self Storage Facility | FL
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	82
Debt	$ 325,016
Land Intial Cost	141,187
Building and Improvements Intial Cost	532,262
Adjustments and Costs Subsequent to Acquisition	41,778
Gross carrying amount - Land	142,081
Gross carrying amount - Building and improvements	573,146
Gross carrying amount - Total	$ 715,227
Self Storage Facility | GA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	55
Debt	$ 124,373
Land Intial Cost	70,611
Building and Improvements Intial Cost	334,343
Adjustments and Costs Subsequent to Acquisition	19,789
Gross carrying amount - Land	70,602
Gross carrying amount - Building and improvements	354,141
Gross carrying amount - Total	$ 424,743
Self Storage Facility | HI
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	9
Debt	$ 39,041
Land Intial Cost	17,663
Building and Improvements Intial Cost	133,870
Adjustments and Costs Subsequent to Acquisition	4,703
Gross carrying amount - Land	17,663
Gross carrying amount - Building and improvements	138,573
Gross carrying amount - Total	$ 156,236
Self Storage Facility | IL
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	31
Debt	$ 75,753
Land Intial Cost	44,427
Building and Improvements Intial Cost	225,423
Adjustments and Costs Subsequent to Acquisition	19,708
Gross carrying amount - Land	43,449
Gross carrying amount - Building and improvements	246,109
Gross carrying amount - Total	$ 289,558
Self Storage Facility | IN
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	15
Debt	$ 16,511
Land Intial Cost	12,447
Building and Improvements Intial Cost	58,247
Adjustments and Costs Subsequent to Acquisition	4,728
Gross carrying amount - Land	12,447
Gross carrying amount - Building and improvements	62,975
Gross carrying amount - Total	$ 75,422
Self Storage Facility | KS
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	1
Debt	$ 0
Land Intial Cost	366
Building and Improvements Intial Cost	1,897
Adjustments and Costs Subsequent to Acquisition	491
Gross carrying amount - Land	366
Gross carrying amount - Building and improvements	2,388
Gross carrying amount - Total	$ 2,754
Self Storage Facility | KY
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	10
Debt	$ 31,023
Land Intial Cost	7,914
Building and Improvements Intial Cost	61,852
Adjustments and Costs Subsequent to Acquisition	4,099
Gross carrying amount - Land	7,914
Gross carrying amount - Building and improvements	65,951
Gross carrying amount - Total	$ 73,865
Self Storage Facility | LA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	2
Debt	$ 8,731
Land Intial Cost	6,114
Building and Improvements Intial Cost	8,541
Adjustments and Costs Subsequent to Acquisition	1,252
Gross carrying amount - Land	6,115
Gross carrying amount - Building and improvements	9,792
Gross carrying amount - Total	$ 15,907
Self Storage Facility | MA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	41
Debt	$ 109,919
Land Intial Cost	61,040
Building and Improvements Intial Cost	217,696
Adjustments and Costs Subsequent to Acquisition	34,110
Gross carrying amount - Land	61,221
Gross carrying amount - Building and improvements	251,625
Gross carrying amount - Total	$ 312,846
Self Storage Facility | MD
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	32
Debt	$ 144,812
Land Intial Cost	99,147
Building and Improvements Intial Cost	284,253
Adjustments and Costs Subsequent to Acquisition	12,225
Gross carrying amount - Land	98,419
Gross carrying amount - Building and improvements	297,206
Gross carrying amount - Total	$ 395,625
Self Storage Facility | MI
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	6
Debt	$ 2,785
Land Intial Cost	7,657
Building and Improvements Intial Cost	38,777
Adjustments and Costs Subsequent to Acquisition	1,865
Gross carrying amount - Land	7,657
Gross carrying amount - Building and improvements	40,642
Gross carrying amount - Total	$ 48,299
Self Storage Facility | MN
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	1
Debt	$ 0
Land Intial Cost	1,528
Building and Improvements Intial Cost	16,030
Adjustments and Costs Subsequent to Acquisition	240
Gross carrying amount - Land	1,528
Gross carrying amount - Building and improvements	16,270
Gross carrying amount - Total	$ 17,798
Self Storage Facility | MO
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	5
Debt	$ 14,671
Land Intial Cost	4,129
Building and Improvements Intial Cost	15,444
Adjustments and Costs Subsequent to Acquisition	2,977
Gross carrying amount - Land	4,086
Gross carrying amount - Building and improvements	18,464
Gross carrying amount - Total	$ 22,550
Self Storage Facility | MS
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	3
Debt	$ 0
Land Intial Cost	2,420
Building and Improvements Intial Cost	20,849
Adjustments and Costs Subsequent to Acquisition	1,338
Gross carrying amount - Land	2,420
Gross carrying amount - Building and improvements	22,187
Gross carrying amount - Total	$ 24,607
Self Storage Facility | NC
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	16
Debt	$ 30,490
Land Intial Cost	28,298
Building and Improvements Intial Cost	91,659
Adjustments and Costs Subsequent to Acquisition	3,300
Gross carrying amount - Land	28,296
Gross carrying amount - Building and improvements	94,961
Gross carrying amount - Total	$ 123,257
Self Storage Facility | NH
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	2
Debt	$ 6,109
Land Intial Cost	754
Building and Improvements Intial Cost	4,054
Adjustments and Costs Subsequent to Acquisition	1,011
Gross carrying amount - Land	817
Gross carrying amount - Building and improvements	5,002
Gross carrying amount - Total	$ 5,819
Self Storage Facility | NJ
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	55
Debt	$ 206,391
Land Intial Cost	117,000
Building and Improvements Intial Cost	490,627
Adjustments and Costs Subsequent to Acquisition	29,396
Gross carrying amount - Land	117,447
Gross carrying amount - Building and improvements	519,576
Gross carrying amount - Total	$ 637,023
Self Storage Facility | NM
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	10
Debt	$ 11,513
Land Intial Cost	22,889
Building and Improvements Intial Cost	61,575
Adjustments and Costs Subsequent to Acquisition	3,521
Gross carrying amount - Land	22,889
Gross carrying amount - Building and improvements	65,096
Gross carrying amount - Total	$ 87,985
Self Storage Facility | NV
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	14
Debt	$ 28,990
Land Intial Cost	15,252
Building and Improvements Intial Cost	74,376
Adjustments and Costs Subsequent to Acquisition	3,562
Gross carrying amount - Land	15,252
Gross carrying amount - Building and improvements	77,938
Gross carrying amount - Total	$ 93,190
Self Storage Facility | NY
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	22
Debt	$ 117,779
Land Intial Cost	121,479
Building and Improvements Intial Cost	232,875
Adjustments and Costs Subsequent to Acquisition	20,664
Gross carrying amount - Land	122,215
Gross carrying amount - Building and improvements	252,803
Gross carrying amount - Total	$ 375,018
Self Storage Facility | OH
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	16
Debt	$ 37,286
Land Intial Cost	16,677
Building and Improvements Intial Cost	40,923
Adjustments and Costs Subsequent to Acquisition	5,295
Gross carrying amount - Land	16,676
Gross carrying amount - Building and improvements	46,219
Gross carrying amount - Total	$ 62,895
Self Storage Facility | OR
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	6
Debt	$ 31,070
Land Intial Cost	7,906
Building and Improvements Intial Cost	39,576
Adjustments and Costs Subsequent to Acquisition	1,116
Gross carrying amount - Land	7,906
Gross carrying amount - Building and improvements	40,692
Gross carrying amount - Total	$ 48,598
Self Storage Facility | PA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	16
Debt	$ 29,140
Land Intial Cost	22,176
Building and Improvements Intial Cost	123,544
Adjustments and Costs Subsequent to Acquisition	8,138
Gross carrying amount - Land	21,468
Gross carrying amount - Building and improvements	132,390
Gross carrying amount - Total	153,858
Accumulated Depreciation	$ 16,058
Self Storage Facility | RI
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	2
Debt	$ 7,852
Land Intial Cost	3,191
Building and Improvements Intial Cost	6,926
Adjustments and Costs Subsequent to Acquisition	946
Gross carrying amount - Land	3,191
Gross carrying amount - Building and improvements	7,872
Gross carrying amount - Total	11,063
Accumulated Depreciation	$ 2,426
Self Storage Facility | SC
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	23
Debt	$ 44,315
Land Intial Cost	37,075
Building and Improvements Intial Cost	135,760
Adjustments and Costs Subsequent to Acquisition	8,265
Gross carrying amount - Land	37,076
Gross carrying amount - Building and improvements	144,024
Gross carrying amount - Total	181,100
Accumulated Depreciation	$ 14,133
Self Storage Facility | TN
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	17
Debt	$ 48,717
Land Intial Cost	25,938
Building and Improvements Intial Cost	91,497
Adjustments and Costs Subsequent to Acquisition	6,246
Gross carrying amount - Land	25,938
Gross carrying amount - Building and improvements	97,743
Gross carrying amount - Total	123,681
Accumulated Depreciation	$ 12,135
Self Storage Facility | TX
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	97
Debt	$ 291,418
Land Intial Cost	166,643
Building and Improvements Intial Cost	629,982
Adjustments and Costs Subsequent to Acquisition	43,395
Gross carrying amount - Land	166,625
Gross carrying amount - Building and improvements	673,395
Gross carrying amount - Total	840,020
Accumulated Depreciation	$ 71,866
Self Storage Facility | UT
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	10
Debt	$ 21,938
Land Intial Cost	9,008
Building and Improvements Intial Cost	39,295
Adjustments and Costs Subsequent to Acquisition	9,636
Gross carrying amount - Land	9,008
Gross carrying amount - Building and improvements	48,931
Gross carrying amount - Total	57,939
Accumulated Depreciation	$ 7,515
Self Storage Facility | VA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	44
Debt	$ 204,498
Land Intial Cost	132,362
Building and Improvements Intial Cost	390,878
Adjustments and Costs Subsequent to Acquisition	13,872
Gross carrying amount - Land	132,363
Gross carrying amount - Building and improvements	404,749
Gross carrying amount - Total	537,112
Accumulated Depreciation	$ 44,694
Self Storage Facility | DC
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	8
Debt	$ 32,080
Land Intial Cost	12,528
Building and Improvements Intial Cost	47,645
Adjustments and Costs Subsequent to Acquisition	1,791
Gross carrying amount - Land	12,530
Gross carrying amount - Building and improvements	49,434
Gross carrying amount - Total	61,964
Accumulated Depreciation	$ 8,581
Self Storage Facility | DC
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Store Count | store	1
Debt	$ 9,304
Land Intial Cost	14,394
Building and Improvements Intial Cost	18,172
Adjustments and Costs Subsequent to Acquisition	326
Gross carrying amount - Land	14,394
Gross carrying amount - Building and improvements	18,498
Gross carrying amount - Total	32,892
Accumulated Depreciation	812
Other Corporate Assets
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Debt	0
Land Intial Cost	0
Building and Improvements Intial Cost	2,202
Adjustments and Costs Subsequent to Acquisition	97,501
Gross carrying amount - Land	0
Gross carrying amount - Building and improvements	99,703
Gross carrying amount - Total	99,703
Accumulated Depreciation	25,948
Intangible tenant relationships and lease rights
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Debt	0
Land Intial Cost	0
Building and Improvements Intial Cost	126,819
Adjustments and Costs Subsequent to Acquisition	0
Gross carrying amount - Land	0
Gross carrying amount - Building and improvements	126,819
Gross carrying amount - Total	126,819
Accumulated Depreciation	112,347
Construction in Progress/Undeveloped Land
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Debt	0
Land Intial Cost	17,874
Building and Improvements Intial Cost	0
Adjustments and Costs Subsequent to Acquisition	29,275
Gross carrying amount - Land	13,245
Gross carrying amount - Building and improvements	33,904
Gross carrying amount - Total	47,149
Accumulated Depreciation	$ 0