Secured and Unsecured Debt (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013	Dec. 31, 2012
Secured debt instruments
Weighted Average Interest Rate	4.20%
Long-term Debt	$ 3,394,723	$ 3,394,723	$ 3,409,333
Senior Notes	250,000	250,000
Unsecured Debt	2,009,689	2,009,689	1,979,198
Weighted Average Years to Maturity		4 years 2 months 26 days
Line of Credit Facility, Amount Outstanding	839,700	839,700
Fixed Rate Debt [Member]
Secured debt instruments
Principal outstanding	1,078,925	1,078,925	1,086,611
Weighted Average Interest Rate	5.17%
Weighted Average Years to Maturity		4 years 5 months 14 days
Number of Communities Encumbered	30
Variable Rate Debt [Member]
Secured debt instruments
Principal outstanding	306,109	306,109	343,524
Weighted Average Interest Rate	1.40%
Weighted Average Years to Maturity		7 years 11 months 6 days
Number of Communities Encumbered	9
Mortgages Notes Payable [Member] | Fixed Rate Debt [Member]
Secured debt instruments
Principal outstanding	450,641	450,641	455,533
Weighted Average Interest Rate	5.42%
Weighted Average Years to Maturity		3 years 0 months 3 days
Number of Communities Encumbered	8
Mortgages Notes Payable [Member] | Variable Rate Debt [Member]
Secured debt instruments
Principal outstanding	0	0	37,415
Weighted Average Interest Rate	0.00%
Weighted Average Years to Maturity		3 months
Number of Communities Encumbered	0
Tax-exempt secured notes payable [Member] | Variable Rate Debt [Member]
Secured debt instruments
Principal outstanding	94,700	94,700	94,700
Weighted Average Interest Rate	0.89%
Weighted Average Years to Maturity		9 years 8 months 8 days
Number of Communities Encumbered	2
Fannie Mae credit facilities [Member] | Fixed Rate Debt [Member]
Secured debt instruments
Principal outstanding			631,078
Debt, Weighted Average Interest Rate	4.99%	4.99%
Weighted Average Years to Maturity		5 years 6 months 3 days
Number of Communities Encumbered	22
Fannie Mae credit facilities [Member] | Variable Rate Debt [Member]
Secured debt instruments
Principal outstanding	211,409	211,409	211,409
Debt Instrument, Interest Rate at Period End	1.63%	1.63%
Weighted Average Years to Maturity		5 years 0 months 7 days
Number of Communities Encumbered	7
Secured Debt [Member]
Secured debt instruments
Principal outstanding	1,385,034	1,385,034	1,430,135
Weighted Average Interest Rate	4.34%
Long-term Debt	1,385,034	1,385,034
Weighted Average Years to Maturity		5 years 2 months 15 days
Number of Communities Encumbered	39
Secured Debt [Member] | Fixed Rate Debt [Member]
Secured debt instruments
Long-term Debt	1,078,925	1,078,925
Secured Debt [Member] | Variable Rate Debt [Member]
Secured debt instruments
Long-term Debt	306,109	306,109
Unsecured Revolving Credit Facility due October 2015 [Member]
Secured debt instruments
Weighted Average Interest Rate	1.30%
Weighted Average Years to Maturity		4 years 5 months 4 days
Line of Credit Facility, Amount Outstanding			76,000
Four Point Six Three Percent Term Medium Notes Due January Two Thousand Twenty-Two [Member]
Secured debt instruments
Weighted Average Interest Rate	4.63%
Senior Notes	396,939	396,939	396,759
Weighted Average Years to Maturity		8 years 6 months 10 days
1.67% Term Notes due December 2016 [Member]
Secured debt instruments
Weighted Average Interest Rate	1.45%
Senior Notes	35,000	35,000	35,000
Weighted Average Years to Maturity		4 years 11 months 4 days
2.68% Term Notes due December 2016 [Member]
Secured debt instruments
Weighted Average Interest Rate	2.50%
Senior Notes	65,000	65,000	65,000
Weighted Average Years to Maturity		4 years 11 months 4 days
Six Point Zero Five Percent, Medium Term Notes, Due June 2013 [Member]
Secured debt instruments
Weighted Average Interest Rate
Senior Notes	0	0	122,500
Weighted Average Years to Maturity		0 months 0 days
Five Point One Three Percent, Medium Term Notes, Due January 2014 [Member]
Secured debt instruments
Weighted Average Interest Rate	5.13%
Senior Notes	184,000	184,000	184,000
Weighted Average Years to Maturity		6 months 14 days
Five Point Five Zero Percent, Medium Term Notes, Due April 2014 [Member]
Secured debt instruments
Weighted Average Interest Rate	5.50%
Senior Notes	128,446	128,446	128,411
Weighted Average Years to Maturity		9 months
Five Point Two Five Percent, Medium Term Notes, Due January 2015 [Member]
Secured debt instruments
Weighted Average Interest Rate	5.25%
Senior Notes	324,977	324,977	324,913
Weighted Average Years to Maturity		1 year 6 months 14 days
Five Point Two Five Percent, Medium Term Notes, Due January 2016 [Member]
Secured debt instruments
Weighted Average Interest Rate	5.25%
Senior Notes	83,260	83,260	83,260
Weighted Average Years to Maturity		2 years 6 months 14 days
2.90% Term Notes due January 2016 [Member]
Secured debt instruments
Weighted Average Interest Rate	2.73%
Senior Notes	250,000	250,000	250,000
Weighted Average Years to Maturity		4 years 11 months 4 days
Eight Point Five Zero Percent, Debentures, Due September 2024 [Member]
Secured debt instruments
Weighted Average Interest Rate	8.50%
Senior Notes	15,644	15,644	15,644
Weighted Average Years to Maturity		11 years 2 months 15 days
Four Point Two Five Percentage Medium-Term Notes due June 2018 [Member]
Secured debt instruments
Weighted Average Interest Rate	4.25%
Senior Notes	297,892	297,892	297,678
Weighted Average Years to Maturity		4 years 11 months 1 day
Other [Member]
Secured debt instruments
Senior Notes	31	31	33
Unsecured Debt [Member]
Secured debt instruments
Weighted Average Interest Rate	4.10%
Long-term Debt	2,009,689	2,009,689
Unsecured Debt	2,009,689	2,009,689	1,979,198
Weighted Average Years to Maturity		4 years 3 months 29 days
Fair Value, Measurements, Recurring [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member] | Mortgages Notes Payable [Member] | Fannie Mae credit facilities [Member] | Fixed Rate Debt [Member]
Secured debt instruments
Principal outstanding	628,284	628,284
Fair Value, Measurements, Recurring [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member] | Senior Unsecured Notes [Member] | Unsecured Revolving Credit Facility due October 2015 [Member]
Secured debt instruments
Line of Credit Facility, Amount Outstanding	228,500	228,500
Fair Value, Measurements, Recurring [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member] | Line of Credit [Member] | Fannie Mae credit facilities [Member] | Variable Rate Debt [Member]
Secured debt instruments
Principal outstanding	$ 211,409	$ 211,409