Supplemental Disclosures of Cash Flow Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Supplemental Disclosures of Cash Flow Information
Cash paid for interest	$ 198,800,000	$ 190,800,000
Interest capitalized to properties under development	347,000	344,000
Cash paid for income taxes	$ 4,000,000	3,600,000
Non-cash activities
Loans Assumed		32,500,000
Net premiums recorded on mortgages assumed		692,000
Increase in buildings and improvements and accounts payable		$ 1,500,000