Borrowing Arrangements (Mortgages) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Debt assumed in connection with acquisition	$ 241,076,000	$ 183,848,000	$ 412,825,000
Gain on extinguishment of debt	(5,564,000)	(1,201,000)	(37,640,000)
Mortgage loans
Debt Instrument [Line Items]
Number of mortgage loans outstanding	160
Mortgage loans, outstanding balance	2,300,000,000
Number of properties which are held as collateral for the mortgage loans	178
Number of mortgage loans bearing interest at fixed rates	143
Principal amount outstanding of fixed rate mortgage loans	1,800,000,000
Number of mortgage loans bearing interest at lender's variable rates	17
Principal amount outstanding of variable rate mortgage loans	474,000,000
Weighted average annual rate on fixed rate mortgage loans (as a percent)	5.90%
Weighted average annual rate on variable rate mortgage loans (as a percent)	2.30%
Weighted average maturity period of mortgage loans (in years)	5 years 7 months 9 days
Debt assumed in connection with acquisition	246,800,000	178,800,000	380,300,000
Mortgage loans repaid in full	398,000,000	493,700,000	344,200,000
Gain on extinguishment of debt	$ 2,300,000	$ 500,000	$ 2,100,000
Mortgage loans | Minimum
Debt Instrument [Line Items]
Fixed interest rate (as a percent)	3.60%
Interest rate on mortgage loans bearing interest at lender's variable rates (as a percent)	0.70%
Mortgage loans | Maximum
Debt Instrument [Line Items]
Fixed interest rate (as a percent)	8.60%
Interest rate on mortgage loans bearing interest at lender's variable rates (as a percent)	3.50%