Commitments (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments
Rent expense	$ 420,000	$ 411,000	$ 408,000
Maximum amount of loan	79,500,000
Amount drawn and outstanding	71,720,000	$ 81,780,000
Office buildings and industrial properties
Future minimum rental income:
2017	180,321,000
2018	156,530,000
2019	139,250,000
2020	121,185,000
2021	92,803,000
Thereafter (2021-2029)	244,075,000
Total	$ 934,164,000
Office buildings and industrial properties | Minimum
Commitments
Lease term	1 year
Corporate office space
Commitments
Extension period	5 years
Future minimum lease payments:
2017	$ 404,000
2018	412,000
2019	421,000
2020	429,000
2021	438,000
Thereafter	787,000
Total	$ 2,891,000