SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION (Details) - USD ($) $ in Thousands		12 Months Ended
	Mar. 01, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION
Encumbrances		$ 388,100
Initial Cost
Land		266,410
Building and Improvements		944,725
Net Improvements (Write-Offs) Since Acquisition		133,501
Gross Amount at Which Carried
Land		266,410
Building and Improvements		1,078,226
Investments in real estate		1,344,636	$ 1,228,780
Accumulated Depreciation		303,699	271,055	$ 414,552	$ 369,661
Other disclosures
Payment of mortgages payable		0	65,877	$ 4,354
Federal tax cost basis (unaudited)		1,351,185
Office | 830 1st Street Washington, DC
SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION
Encumbrances		46,000
Initial Cost
Land		18,095
Building and Improvements		62,017
Net Improvements (Write-Offs) Since Acquisition		(1,106)
Gross Amount at Which Carried
Land		18,095
Building and Improvements		60,911
Investments in real estate		79,006
Accumulated Depreciation		19,467
Office | 3601 S Congress Avenue Austin, TX
SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION
Encumbrances		0
Initial Cost
Land		9,569
Building and Improvements		18,593
Net Improvements (Write-Offs) Since Acquisition		3,257
Gross Amount at Which Carried
Land		9,569
Building and Improvements		21,850
Investments in real estate		31,419
Accumulated Depreciation		7,834
Office | 899 N Capitol Street Washington, DC
SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION
Encumbrances		0
Initial Cost
Land		34,641
Building and Improvements		84,466
Net Improvements (Write-Offs) Since Acquisition		15,492
Gross Amount at Which Carried
Land		34,641
Building and Improvements		99,958
Investments in real estate		134,599
Accumulated Depreciation		33,368
Office | 999 N Capitol Street Washington, DC
SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION
Encumbrances		0
Initial Cost
Land		32,221
Building and Improvements		86,526
Net Improvements (Write-Offs) Since Acquisition		14,654
Gross Amount at Which Carried
Land		32,221
Building and Improvements		101,180
Investments in real estate		133,401
Accumulated Depreciation		34,159
Office | 901 N Capitol Street Washington, DC
SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION
Encumbrances		0
Initial Cost
Land		27,117
Building and Improvements		0
Net Improvements (Write-Offs) Since Acquisition		4,333
Gross Amount at Which Carried
Land		27,117
Building and Improvements		4,333
Investments in real estate		31,450
Accumulated Depreciation		1,483
Office | 1333 Broadway Oakland, CA
SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION
Encumbrances		39,500
Initial Cost
Land		7,048
Building and Improvements		41,578
Net Improvements (Write-Offs) Since Acquisition		15,120
Gross Amount at Which Carried
Land		7,048
Building and Improvements		56,698
Investments in real estate		63,746
Accumulated Depreciation		15,946
Office | 1901 Harrison Street Oakland, CA
SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION
Encumbrances		42,500
Initial Cost
Land		3,838
Building and Improvements		68,106
Net Improvements (Write-Offs) Since Acquisition		10,308
Gross Amount at Which Carried
Land		3,838
Building and Improvements		78,414
Investments in real estate		82,252
Accumulated Depreciation		25,646
Office | 2100 Franklin Street Oakland, CA
SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION
Encumbrances		80,000
Initial Cost
Land		4,277
Building and Improvements		34,033
Net Improvements (Write-Offs) Since Acquisition		10,200
Gross Amount at Which Carried
Land		4,277
Building and Improvements		44,233
Investments in real estate		48,510
Accumulated Depreciation		16,374
Office | 2101 Webster Street Oakland, CA
SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION
Encumbrances		83,000
Initial Cost
Land		4,752
Building and Improvements		109,812
Net Improvements (Write-Offs) Since Acquisition		32,006
Gross Amount at Which Carried
Land		4,752
Building and Improvements		141,818
Investments in real estate		146,570
Accumulated Depreciation		46,872
Office | 2353 Webster Street Parking Garage Oakland, CA
SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION
Encumbrances		0
Initial Cost
Land		0
Building and Improvements		9,138
Net Improvements (Write-Offs) Since Acquisition		27
Gross Amount at Which Carried
Land		0
Building and Improvements		9,165
Investments in real estate		9,165
Accumulated Depreciation		2,353
Office | 1 Kaiser Plaza Oakland, CA
SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION
Encumbrances		97,100
Initial Cost
Land		9,261
Building and Improvements		113,619
Net Improvements (Write-Offs) Since Acquisition		18,224
Gross Amount at Which Carried
Land		9,261
Building and Improvements		131,843
Investments in real estate		141,104
Accumulated Depreciation		39,832
Office | 2 Kaiser Plaza Parking Lot Oakland, CA
SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION
Encumbrances		0
Initial Cost
Land		10,931
Building and Improvements		110
Net Improvements (Write-Offs) Since Acquisition		1,714
Gross Amount at Which Carried
Land		10,931
Building and Improvements		1,824
Investments in real estate		12,755
Accumulated Depreciation		86
Office | 11600 Wilshire Blvd Los Angeles, CA
SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION
Encumbrances		0
Initial Cost
Land		3,477
Building and Improvements		18,522
Net Improvements (Write-Offs) Since Acquisition		2,266
Gross Amount at Which Carried
Land		3,477
Building and Improvements		20,788
Investments in real estate		24,265
Accumulated Depreciation		5,133
Office | 11620 Wilshire Blvd Los Angeles, CA
SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION
Encumbrances		0
Initial Cost
Land		7,672
Building and Improvements		51,999
Net Improvements (Write-Offs) Since Acquisition		8,690
Gross Amount at Which Carried
Land		7,672
Building and Improvements		60,689
Investments in real estate		68,361
Accumulated Depreciation		15,583
Office | 4750 Wilshire Blvd Los Angeles, CA
SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION
Encumbrances		0
Initial Cost
Land		16,633
Building and Improvements		28,985
Net Improvements (Write-Offs) Since Acquisition		105
Gross Amount at Which Carried
Land		16,633
Building and Improvements		29,090
Investments in real estate		45,723
Accumulated Depreciation		4,073
Office | Lindblade Media Center Los Angeles, CA
SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION
Encumbrances		0
Initial Cost
Land		6,342
Building and Improvements		11,568
Net Improvements (Write-Offs) Since Acquisition		(2)
Gross Amount at Which Carried
Land		6,342
Building and Improvements		11,566
Investments in real estate		17,908
Accumulated Depreciation		1,623
Office | 1130 Howard Street San Francisco, CA
SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION
Encumbrances		0
Initial Cost
Land		8,290
Building and Improvements		10,480
Net Improvements (Write-Offs) Since Acquisition		5
Gross Amount at Which Carried
Land		8,290
Building and Improvements		10,485
Investments in real estate		18,775
Accumulated Depreciation		315
Office | 9460 Wilshire Boulevard Los Angeles, CA
SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION
Encumbrances		0
Initial Cost
Land		52,199
Building and Improvements		76,730
Net Improvements (Write-Offs) Since Acquisition		65
Gross Amount at Which Carried
Land		52,199
Building and Improvements		76,795
Investments in real estate		128,994
Accumulated Depreciation		2,440
Hotel | Sheraton Grand Hotel Sacramento, CA
SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION
Encumbrances		0
Initial Cost
Land		3,497
Building and Improvements		107,447
Net Improvements (Write-Offs) Since Acquisition		(1,934)
Gross Amount at Which Carried
Land		3,497
Building and Improvements		105,513
Investments in real estate		109,010
Accumulated Depreciation		28,158
Hotel | Sheraton Grand Hotel Parking & Retail Sacramento, CA
SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION
Encumbrances		0
Initial Cost
Land		6,550
Building and Improvements		10,996
Net Improvements (Write-Offs) Since Acquisition		77
Gross Amount at Which Carried
Land		6,550
Building and Improvements		11,073
Investments in real estate		17,623
Accumulated Depreciation		2,954
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Subsequent event | Office Property, Washington, D.C
Other disclosures
Payment of mortgages payable	$ 46,000
Line of Credit | Revolving Credit Facility
Other disclosures
Outstanding balance		130,000	0
Mortgage loans with a fixed interest rate of 4.14% per annum
Other disclosures
Outstanding balance		$ 342,100	$ 370,300
Mortgage loans with a fixed interest rate of 4.14% per annum | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Subsequent event | Properties Used As Collateral For Loans
Other disclosures
Defeased amount	$ 205,500