Fair Value of Assets and Liabilities - Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value of Financial Instruments
Senior Notes	$ 945,948	$ 944,140
Mortgage notes payable, net	$ 176,828	183,100
Senior unsecured notes, 3.75% interest rate, due in 2019
Fair Value of Financial Instruments
Interest rate (as a percent)	3.75%
Senior unsecured notes, 5.875% interest rate, due in 2046
Fair Value of Financial Instruments
Interest rate (as a percent)	5.875%
Senior unsecured notes, 4.000% interest rate, due in 2022
Fair Value of Financial Instruments
Interest rate (as a percent)	4.00%
Mortgage note payable, 4.050% interest rate, due in 2030
Fair Value of Financial Instruments
Interest rate (as a percent)	4.05%
Mortgage note payable, 5.720% interest rate, due in 2020
Fair Value of Financial Instruments
Interest rate (as a percent)	5.72%
Mortgage note payable, 4.220% interest rate, due in 2022
Fair Value of Financial Instruments
Interest rate (as a percent)	4.22%
Mortgage note payable, 4.800% interest rate, due in 2023
Fair Value of Financial Instruments
Interest rate (as a percent)	4.80%
Mortgage note payable, 5.877% interest rate, due in 2021
Fair Value of Financial Instruments
Interest rate (as a percent)	5.877%
Mortgage note payable, 7.000% interest rate, due in 2019
Fair Value of Financial Instruments
Interest rate (as a percent)	7.00%
Mortgage note payable, 8.150% interest rate, due in 2021
Fair Value of Financial Instruments
Interest rate (as a percent)	8.15%
Mortgage note payable, 4.260% interest rate, due in 2020
Fair Value of Financial Instruments
Interest rate (as a percent)	4.26%
Carrying Amount
Fair Value of Financial Instruments
Fair value of financial instruments	$ 1,125,823	1,127,240
Carrying Amount \| Senior unsecured notes, 3.75% interest rate, due in 2019
Fair Value of Financial Instruments
Senior Notes	348,953	348,096
Carrying Amount \| Senior unsecured notes, 5.875% interest rate, due in 2046
Fair Value of Financial Instruments
Senior Notes	300,490	300,232
Carrying Amount \| Senior unsecured notes, 4.000% interest rate, due in 2022
Fair Value of Financial Instruments
Senior Notes	296,505	295,812
Carrying Amount \| Mortgage note payable, 4.050% interest rate, due in 2030
Fair Value of Financial Instruments
Mortgage notes payable, net	64,441	64,293
Carrying Amount \| Mortgage note payable, 5.720% interest rate, due in 2020
Fair Value of Financial Instruments
Mortgage notes payable, net	35,067	36,085
Carrying Amount \| Mortgage note payable, 4.220% interest rate, due in 2022
Fair Value of Financial Instruments
Mortgage notes payable, net	27,408	27,906
Carrying Amount \| Mortgage note payable, 4.800% interest rate, due in 2023
Fair Value of Financial Instruments
Mortgage notes payable, net	25,145	25,501
Carrying Amount \| Mortgage note payable, 5.877% interest rate, due in 2021
Fair Value of Financial Instruments
Mortgage notes payable, net	13,445	13,620
Carrying Amount \| Mortgage note payable, 7.000% interest rate, due in 2019
Fair Value of Financial Instruments
Mortgage notes payable, net	8,087	8,391
Carrying Amount \| Mortgage note payable, 8.150% interest rate, due in 2021
Fair Value of Financial Instruments
Mortgage notes payable, net	3,235	4,111
Carrying Amount \| Mortgage note payable, 4.260% interest rate, due in 2020
Fair Value of Financial Instruments
Mortgage notes payable, net	3,047	3,193
Fair Value
Fair Value of Financial Instruments
Fair value of financial instruments	1,135,834	1,164,606
Fair Value \| Senior unsecured notes, 3.75% interest rate, due in 2019
Fair Value of Financial Instruments
Senior Notes	351,468	354,993
Fair Value \| Senior unsecured notes, 5.875% interest rate, due in 2046
Fair Value of Financial Instruments
Senior Notes	309,380	320,416
Fair Value \| Senior unsecured notes, 4.000% interest rate, due in 2022
Fair Value of Financial Instruments
Senior Notes	296,609	302,655
Fair Value \| Mortgage note payable, 4.050% interest rate, due in 2030
Fair Value of Financial Instruments
Mortgage notes payable, net	62,363	65,198
Fair Value \| Mortgage note payable, 5.720% interest rate, due in 2020
Fair Value of Financial Instruments
Mortgage notes payable, net	34,877	36,332
Fair Value \| Mortgage note payable, 4.220% interest rate, due in 2022
Fair Value of Financial Instruments
Mortgage notes payable, net	27,422	28,432
Fair Value \| Mortgage note payable, 4.800% interest rate, due in 2023
Fair Value of Financial Instruments
Mortgage notes payable, net	25,220	25,904
Fair Value \| Mortgage note payable, 5.877% interest rate, due in 2021
Fair Value of Financial Instruments
Mortgage notes payable, net	13,978	14,565
Fair Value \| Mortgage note payable, 7.000% interest rate, due in 2019
Fair Value of Financial Instruments
Mortgage notes payable, net	8,129	8,555
Fair Value \| Mortgage note payable, 8.150% interest rate, due in 2021
Fair Value of Financial Instruments
Mortgage notes payable, net	3,360	4,340
Fair Value \| Mortgage note payable, 4.260% interest rate, due in 2020
Fair Value of Financial Instruments
Mortgage notes payable, net	$ 3,028	$ 3,216

Contractual interest rate for funds borrowed, under the debt agreement.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-Section S99
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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-Section S99
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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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