SECURED AND UNSECURED DEBT, NET - Summary (Details) $ in Thousands	12 Months Ended
Dec. 31, 2018 USD ($) community item	Dec. 31, 2017 USD ($)
Secured debt instruments
Unamortized discount	$ 5,000	$ 8,200
Interest rate at end of the period	3.72%
Long-term Debt	$ 3,547,787	3,671,663
Unsecured Debt	2,946,560	2,868,394
Borrowings outstanding	$ 3,300	3,300
Number of secured credit facilities with Fannie Mae \| item	1
Weighted Average
Secured debt instruments
Years to maturity	5 years 9 months 18 days
Commercial Paper
Secured debt instruments
Interest rate at end of the period	2.90%
Borrowings outstanding at end of period	$ 101,115	300,000
Borrowings outstanding	$ 101,115	300,000
Commercial Paper \| Weighted Average
Secured debt instruments
Years to maturity	1 month 6 days
Mortgages Notes Payable \| Fixed Rate Debt
Secured debt instruments
Principal outstanding	$ 417,989	395,611
Interest rate at end of the period	3.82%
Number of Communities Encumbered \| community	7
Mortgages Notes Payable \| Fixed Rate Debt \| Weighted Average
Secured debt instruments
Years to maturity	5 years 10 months 24 days
Tax-exempt secured notes payable \| Variable Rate Debt
Secured debt instruments
Principal outstanding	$ 94,700	94,700
Interest rate at end of the period	2.33%
Number of Communities Encumbered \| community	2
Tax-exempt secured notes payable \| Variable Rate Debt \| Weighted Average
Secured debt instruments
Years to maturity	4 years 2 months 12 days
Fannie Mae credit facilities \| Variable Rate Debt
Secured debt instruments
Principal outstanding		29,034
Secured Debt
Secured debt instruments
Long-term Debt	$ 601,227
Secured Debt \| Fixed Rate Debt
Secured debt instruments
Principal outstanding	$ 506,646	679,777
Interest rate at end of the period	3.85%
Long-term Debt	$ 506,646
Number of Communities Encumbered \| community	8
Secured Debt \| Fixed Rate Debt \| Weighted Average
Secured debt instruments
Years to maturity	5 years 1 month 6 days
Secured Debt \| Variable Rate Debt
Secured debt instruments
Long-term Debt	$ 94,581
Deferred finance costs, net	$ (119)	(242)
Unsecured Revolving credit facility due 2023 \| Weighted Average
Secured debt instruments
Years to maturity	4 years 1 month 6 days
Unsecured Working Capital Credit Facility due January 2019
Secured debt instruments
Borrowings outstanding	$ 16	21,767
Unsecured Working Capital Credit Facility due January 2021
Secured debt instruments
Interest rate at end of the period	3.33%
Borrowings outstanding	$ 16	21,767
Unsecured Working Capital Credit Facility due January 2021 \| Weighted Average
Secured debt instruments
Years to maturity	2 years
Term Loan due September 2023
Secured debt instruments
Interest rate at end of the period	3.25%
Senior Notes	$ 35,000	35,000
Term Loan due September 2023 \| Weighted Average
Secured debt instruments
Years to maturity	4 years 9 months 18 days
3.70% Term Notes Due October 2020
Secured debt instruments
Unamortized discount	$ 14	22
Interest rate at end of the period	3.70%
Senior Notes	$ 299,986	299,978
3.70% Term Notes Due October 2020 \| Weighted Average
Secured debt instruments
Years to maturity	1 year 9 months 18 days
4.63% Medium-Term Notes due January 2022
Secured debt instruments
Unamortized discount	$ 1,087	1,446
Interest rate at end of the period	4.63%
Senior Notes	$ 398,913	398,554
4.63% Medium-Term Notes due January 2022 \| Weighted Average
Secured debt instruments
Years to maturity	3 years
1.93% Term Loan due September 2023
Secured debt instruments
Interest rate at end of the period	1.93%
Senior Notes	$ 315,000	315,000
1.93% Term Loan due September 2023 \| Weighted Average
Secured debt instruments
Years to maturity	4 years 9 months 18 days
3.75% Medium-Term Notes Due July 2024
Secured debt instruments
Unamortized discount	$ 574	678
Interest rate at end of the period	3.75%
Senior Notes	$ 299,426	299,322
3.75% Medium-Term Notes Due July 2024 \| Weighted Average
Secured debt instruments
Years to maturity	5 years 6 months
8.50% Debentures, Due September 2024
Secured debt instruments
Interest rate at end of the period	8.50%
Senior Notes	$ 15,644	15,644
8.50% Debentures, Due September 2024 \| Weighted Average
Secured debt instruments
Years to maturity	5 years 8 months 12 days
4.00% Medium-Term Note due October 2025
Secured debt instruments
Unamortized discount	$ 465	534
Interest rate at end of the period	4.00%
Senior Notes	$ 299,535	299,466
4.00% Medium-Term Note due October 2025 \| Weighted Average
Secured debt instruments
Years to maturity	6 years 9 months 18 days
2.95% Medium-Term Note due September 2026
Secured debt instruments
Interest rate at end of the period	2.95%
Senior Notes	$ 300,000	300,000
2.95% Medium-Term Note due September 2026 \| Weighted Average
Secured debt instruments
Years to maturity	7 years 8 months 12 days
3.50 Medium-Term Note due July 2027
Secured debt instruments
Unamortized discount	$ 600	670
Interest rate at end of the period	3.50%
Senior Notes	$ 299,400	299,330
3.50 Medium-Term Note due July 2027 \| Weighted Average
Secured debt instruments
Years to maturity	8 years 6 months
3.50% Medium-Term Notes Due January 2028
Secured debt instruments
Unamortized discount	$ 1,072	1,191
Interest rate at end of the period	3.50%
Senior Notes	$ 298,928	298,809
3.50% Medium-Term Notes Due January 2028 \| Weighted Average
Secured debt instruments
Years to maturity	9 years
4.40% Medium-Term Notes due January 2029
Secured debt instruments
Unamortized discount	$ 6	0
Interest rate at end of the period	4.40%
Senior Notes	$ 299,994
4.40% Medium-Term Notes due January 2029 \| Weighted Average
Secured debt instruments
Years to maturity	10 years 1 month 6 days
Other
Secured debt instruments
Senior Notes	$ 16	19
Unsecured Debt
Secured debt instruments
Interest rate at end of the period	3.65%
Long-term Debt	$ 2,946,560
Unsecured Debt	2,946,560	2,868,394
Deferred finance costs, net	$ (16,413)	(14,495)
Unsecured Debt \| Weighted Average
Secured debt instruments
Years to maturity	6 years
Unsecured Revolving Credit Facility due October 2015 \| Commercial Paper
Secured debt instruments
Borrowings outstanding	$ 101,100	300,000
Secured Debt
Secured debt instruments
Principal outstanding	$ 601,227	803,269
Interest rate at end of the period	3.61%
Number of Communities Encumbered \| community	10
Secured Debt \| Weighted Average
Secured debt instruments
Years to maturity	5 years
Secured Debt \| Fixed Rate Debt
Secured debt instruments
Deferred finance costs, net	$ (1,343)	(1,670)
Secured Debt \| Variable Rate Debt
Secured debt instruments
Principal outstanding	$ 94,581	123,492
Interest rate at end of the period	2.33%
Number of Communities Encumbered \| community	2
Secured Debt \| Variable Rate Debt \| Weighted Average
Secured debt instruments
Years to maturity	4 years 2 months 12 days
Fannie Mae credit facilities \| Fixed Rate Debt
Secured debt instruments
Principal outstanding	$ 90,000	$ 285,836
Interest rate at end of the period	3.95%
Number of Communities Encumbered \| community	1
Fannie Mae credit facilities \| Fixed Rate Debt \| Weighted Average
Secured debt instruments
Years to maturity	1 year 6 months

Number of Communities Encumbered.

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- Definition

The number of secured credit facilities with Fannie Mae.

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No definition available.

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- Definition

Secured debt including debt on real estate held for sale.

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No definition available.

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- Definition

Unsecured short-term debt instrument issued by corporations which are highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

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- Definition

The average effective interest rate during the reporting period.

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- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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No definition available.

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- Definition

Amount, after accumulated amortization, of debt discount.

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- Definition

Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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- Definition

Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of uncollateralized debt obligations (with maturities initially due after one year or beyond the operating cycle if longer).

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