Supplemental Disclosures of Cash Flow Information (Details)	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($) property
Non-cash investing and financing activities
Cash paid for interest	$ 214,300,000	$ 229,500,000	$ 207,300,000
Interest capitalized to properties under development	469,000	594,000	444,000
Cash paid for income taxes	3,600,000	$ 3,100,000	3,700,000
Loans Assumed	44,100,000		166,700,000
Net premiums recorded on mortgages assumed	692,000		604,000
Contributions	15,906,000
Interest rate swap agreement acquired			901,000
Excess of redemption value over carrying value of preferred shares redeemed			6,015,000
Loans receivable			$ 48,900,000
Number of acquired properties | property			5
Real estate acquired in exchange of properties			$ 11,600,000
Increase in Buildings and Improvements and Accounts Payable	$ 2,600,000		$ 4,000,000