Debt (Narrative) (Details) (USD $)	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Nov. 14, 2014		Dec. 12, 2014	Jan. 02, 2014
Debt Instrument [Line Items]
Net proceeds from note offering	$ 244,579,000	$ 564,389,000	$ 244,807,000
Maximum amount of borrowings outstanding	79,500,000	76,000,000	186,000,000
Weighted average amount of borrowings outstanding	12,500,000	10,500,000	1,500,000
Weighted average interest rate before amortization of debt fees	1.10%	1.30%	1.40%
Annual facility fee	900,000
Revolving credit facility, max capacity	600,000,000
4.50% notes
Debt Instrument [Line Items]
Principal balance				250,000,000	[1]
Net proceeds from note offering				244,600,000
Revolving credit facility | Notes Payable
Debt Instrument [Line Items]
Revolving credit facility	0	0
5.65% notes
Debt Instrument [Line Items]
Redemption price						134,500,000
Make-whole premium						9,200,000
Accrued but unpaid interest						200,000
Grove at Shrewsbury and Brook 35
Debt Instrument [Line Items]
Principal balance							68,300,000
Debt instrument, fair value							$ 73,800,000

[1] The net proceeds from this note offering after issuance discounts, underwriting fees and other costs were approximately $244.6 million.