ACQUISITIONS, DEVELOPMENTS PLACED IN SERVICE, AND DISPOSITIONS - Property Dispositions (Details) - Disposed of by Sale [Member] $ in Thousands	3 Months Ended		9 Months Ended
	Jan. 31, 2018 USD ($) ft² property unit	Jan. 31, 2017 USD ($) ft² property	Jan. 31, 2018 USD ($) ft² unit	Jan. 31, 2017 USD ($) ft² property
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale price	$ 442,800	$ 73,900	$ 509,629	$ 131,478
Book value and sales cost			327,571	94,045
Gain/(Loss)			182,058	37,433
Multi-Family Residential [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold | property	2
Sale price			18,913
Book value and sales cost			15,570
Gain/(Loss)			3,343
Multi-Family Residential [Member] | 327 unit Multifamily - Minot, ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale price			12,263
Book value and sales cost			11,562
Gain/(Loss)			$ 701
Units | unit	327		327
Multi-Family Residential [Member] | 48 unit - Crown - Rochester MN [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale price			$ 5,700
Book value and sales cost			3,318
Gain/(Loss)			$ 2,382
Units | unit	48		48
Multi-Family Residential [Member] | 16 unit - Northern Valley - Rochester MN [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale price			$ 950
Book value and sales cost			690
Gain/(Loss)			$ 260
Units | unit	16		16
Multi-Family Residential [Member] | Minot Southgate Retail - Minot, ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Deferred gain on sale of properties	$ 626		$ 626
Healthcare [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold | property	27
Commercial [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold | property	2	6
Other [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale price			487,541	131,228
Book value and sales cost			308,813	93,771
Gain/(Loss)			178,728	37,457
Other [Member] | Minot Southgate Retail - Minot, ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale price			3,440
Book value and sales cost			3,332
Gain/(Loss)			$ 108
Area of real estate property | ft²	4,998		4,998
Other [Member] | Lexington Commerce Center - Eagan, MN [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale price			$ 9,000
Book value and sales cost			3,963
Gain/(Loss)			$ 5,037
Area of real estate property | ft²	90,260		90,260
Other [Member] | Eagan 1440 Duckwood Medical - Eagan, MN [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale price			$ 2,100
Book value and sales cost			1,886
Gain/(Loss)			$ 214
Area of real estate property | ft²	17,640		17,640
Other [Member] | Edgewood Vista - Hermantown, MN [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale price			$ 36,884
Book value and sales cost			24,631
Gain/(Loss)			$ 12,253
Area of real estate property | ft²	279,834		279,834
Other [Member] | Urbandale - Urbandale IA [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale price			$ 16,700
Book value and sales cost			12,857
Gain/(Loss)			$ 3,843
Area of real estate property | ft²	518,161		518,161
Other [Member] | Long Lake Road - Roseville MN [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale price			$ 18,650
Book value and sales cost			12,766
Gain/(Loss)			$ 5,884
Area of real estate property | ft²	36,053		36,053
Other [Member] | Healthcare Properties [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale price			$ 370,267
Book value and sales cost			232,768
Gain/(Loss)			$ 137,499
Area of real estate property | ft²	1,205,432		1,205,432
Escrow Deposit	$ 2,600		$ 2,600
Other [Member] | Garden View - St. Paul MN [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale price			14,000
Book value and sales cost			6,191
Gain/(Loss)			$ 7,809
Area of real estate property | ft²	43,404		43,404
Other [Member] | Ritchie Medical - St. Paul MN [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale price			$ 16,500
Book value and sales cost			10,419
Gain/(Loss)			$ 6,081
Area of real estate property | ft²	52,116		52,116
Other [Member] | Stone Container - Fargo, ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale price				13,400
Book value and sales cost				4,418
Gain/(Loss)				$ 8,982
Area of real estate property | ft²		195,075		195,075
Other [Member] | Idaho Spring Creek Senior Housing Properties [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold | property				8
Sale price				$ 43,900
Book value and sales cost				37,397
Gain/(Loss)				$ 6,503
Area of real estate property | ft²		189,244		189,244
Other [Member] | Grand Forks Carmike - Grand Folks ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale price				$ 4,000
Book value and sales cost				1,563
Gain/(Loss)				$ 2,437
Area of real estate property | ft²		28,652		28,652
Other [Member] | Edgewood Vista Senior Housing Properties [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold | property				5
Sale price				$ 69,928
Book value and sales cost				50,393
Gain/(Loss)				$ 19,535
Area of real estate property | ft²		426,652		426,652
Unimproved Land [Member] | Bismarck 4916 - Bismarck, ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale price			$ 3,175
Book value and sales cost			3,188
Gain/(Loss)			$ (13)
Unimproved Land [Member] | Grand Chute, Wisconsin [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale price				$ 250
Book value and sales cost				274
Gain/(Loss)				$ (24)