Secured Debt and Unsecured Debt (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Unamortized discount	$ 6,700	$ 6,700	$ 11,800
Weighted Average Interest Rate	3.94%
Weighted Average Years to Maturity		4 years 3 months 18 days
Borrowings outstanding	1,900	1,900	2,200
Total Unsecured Debt	2,221,576	2,221,576	2,081,626
Total Debt	3,583,105	3,583,105	3,523,703
Fixed Rate Debt
Debt Instrument [Line Items]
Principal outstanding	969,296	969,296	1,072,373
Weighted Average Interest Rate	5.26%
Weighted Average Years to Maturity		3 years 0 months 2 days
Number of Communities Encumbered		28
Variable Rate Debt
Debt Instrument [Line Items]
Principal outstanding	392,233	392,233	369,704
Weighted Average Interest Rate	1.44%
Weighted Average Years to Maturity		5 years 8 months 7 days
Number of Communities Encumbered		10
Three point seven percent medium term note due October 2020 [Member]
Debt Instrument [Line Items]
Stated interest rate	3.70%	3.70%
Unamortized discount	46	46	54
Secured Debt
Debt Instrument [Line Items]
Principal outstanding	1,361,529	1,361,529	1,442,077
Weighted Average Interest Rate	4.16%
Weighted Average Years to Maturity		3 years 9 months 18 days
Number of Communities Encumbered		38
Total Debt	1,361,529	1,361,529
Secured Debt | Fixed Rate Debt
Debt Instrument [Line Items]
Total Debt	969,296	969,296
Secured Debt | Variable Rate Debt
Debt Instrument [Line Items]
Total Debt	392,233	392,233
Mortgage notes payable | Fixed Rate Debt
Debt Instrument [Line Items]
Principal outstanding			445,706
Weighted Average Interest Rate	5.46%
Weighted Average Years to Maturity		1 year 7 months 20 days
Number of Communities Encumbered	6
Mortgage notes payable | Variable Rate Debt
Debt Instrument [Line Items]
Principal outstanding			63,595
Weighted Average Interest Rate	1.94%
Weighted Average Years to Maturity		2 years 0 months 25 days
Number of Communities Encumbered	1
Tax-exempt secured notes payable | Variable Rate Debt
Debt Instrument [Line Items]
Principal outstanding			94,700
Weighted Average Interest Rate	0.83%
Weighted Average Years to Maturity		8 years 2 months 8 days
Number of Communities Encumbered	2
Fannie Mae credit facilities | Fixed Rate Debt
Debt Instrument [Line Items]
Principal outstanding			626,667
Debt, Weighted Average Interest Rate	5.12%	5.12%
Weighted Average Years to Maturity		4 years 0 months 10 days
Number of Communities Encumbered	22
Fannie Mae credit facilities | Variable Rate Debt
Debt Instrument [Line Items]
Principal outstanding			211,409
Debt Instrument, Interest Rate at Period End	1.60%	1.60%
Weighted Average Years to Maturity		5 years 2 months 8 days
Number of Communities Encumbered	7
Unsecured Debt
Debt Instrument [Line Items]
Weighted Average Interest Rate	3.81%
Weighted Average Years to Maturity		4 years 7 months 6 days
Total Unsecured Debt	2,221,576	2,221,576	2,081,626
Total Debt	2,221,576	2,221,576
Borrowings outstanding under an unsecured credit facility due October 2015
Debt Instrument [Line Items]
Weighted Average Interest Rate	1.09%
Weighted Average Years to Maturity		2 years 11 months 4 days
Borrowings outstanding	152,500	152,500	0
ThreePointSevenTermNotesDueOctober2020 [Member]
Debt Instrument [Line Items]
Weighted Average Years to Maturity		5 years 9 months
Senior Unsecured Notes	299,954	299,954	299,946
4.63% Medium-Term Notes due January 2022
Debt Instrument [Line Items]
Stated interest rate	4.63%	4.63%	4.63%
Unamortized discount	2,523	2,523	2,882
Weighted Average Interest Rate	4.63%
Weighted Average Years to Maturity		7 years 0 months 10 days
Senior Unsecured Notes	397,477	397,477	397,118
3.75 MTN Due July 2024 [Member]
Debt Instrument [Line Items]
Stated interest rate	3.75%	3.75%
Unamortized discount	990	990
Weighted Average Interest Rate	3.75%
Weighted Average Years to Maturity		9 years 6 months
Senior Unsecured Notes	299,010	299,010	0
1.67% Term Notes due December 2016
Debt Instrument [Line Items]
Stated interest rate	1.31%	1.31%	1.65%
Weighted Average Interest Rate	1.31%
Weighted Average Years to Maturity		3 years 5 months 4 days
Senior Unsecured Notes	35,000	35,000	35,000
2.68% Term Notes due December 2016
Debt Instrument [Line Items]
Stated interest rate	1.53%	1.53%	2.68%
Weighted Average Interest Rate	1.53%
Weighted Average Years to Maturity		3 years 5 months 4 days
Senior Unsecured Notes	100,000	100,000	65,000
5.13% Medium-Term Notes due January 2014
Debt Instrument [Line Items]
Stated interest rate	5.13%	5.13%	5.13%
Weighted Average Interest Rate	0.00%
Weighted Average Years to Maturity		1 day
Senior Unsecured Notes	0	0	184,000
5.50% Medium-Term Notes due April 2014
Debt Instrument [Line Items]
Stated interest rate	5.50%	5.50%	5.50%
Unamortized discount	0	0	20
Weighted Average Interest Rate	0.00%
Weighted Average Years to Maturity		0 days
Senior Unsecured Notes	0	0	128,480
5.25% Medium-Term Notes due January 2015
Debt Instrument [Line Items]
Stated interest rate	5.25%	5.25%	5.25%
Unamortized discount	6	6	134
Weighted Average Interest Rate	5.25%
Weighted Average Years to Maturity		14 days
Senior Unsecured Notes	325,169	325,169	325,041
5.25% Medium-Term Notes due January 2016
Debt Instrument [Line Items]
Stated interest rate	5.25%	5.25%	5.25%
Weighted Average Interest Rate	5.25%
Weighted Average Years to Maturity		1 year 0 months 14 days
Senior Unsecured Notes	83,260	83,260	83,260
2.90% Term Notes due January 2016
Debt Instrument [Line Items]
Stated interest rate	2.17%	2.17%	2.90%
Weighted Average Interest Rate	2.17%
Weighted Average Years to Maturity		3 years 5 months 4 days
Senior Unsecured Notes	215,000	215,000	250,000
8.50% Debentures due September 2024
Debt Instrument [Line Items]
Stated interest rate	8.50%	8.50%	8.50%
Weighted Average Interest Rate	8.50%
Weighted Average Years to Maturity		9 years 8 months 15 days
Senior Unsecured Notes	15,644	15,644	15,644
4.25% Medium-Term Notes due June 2018
Debt Instrument [Line Items]
Stated interest rate	4.25%	4.25%	4.25%
Unamortized discount	1,465	1,465	1,893
Weighted Average Interest Rate	4.25%
Weighted Average Years to Maturity		3 years 5 months 4 days
Senior Unsecured Notes	298,535	298,535	298,107
Other
Debt Instrument [Line Items]
Senior Unsecured Notes	$ 27	$ 27	$ 30