Fair Value of Assets and Liabilities - Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value of Financial Instruments
Senior Notes	$ 944,140	$ 646,844
Mortgage notes payable	$ 183,100	27,837
Senior unsecured notes, 3.750% 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
Mortgage notes payable	$ 1,127,240	674,681
Carrying Amount \| Senior unsecured notes, 3.750% interest rate, due in 2019
Fair Value of Financial Instruments
Senior Notes	348,096	346,952
Carrying Amount \| Senior unsecured notes, 5.875% interest rate, due in 2046
Fair Value of Financial Instruments
Senior Notes	300,232	299,892
Carrying Amount \| Senior unsecured notes, 4.000% interest rate, due in 2022
Fair Value of Financial Instruments
Senior Notes	295,812	0
Carrying Amount \| Mortgage note payable, 4.050% interest rate, due in 2030
Fair Value of Financial Instruments
Senior Notes	64,293	0
Carrying Amount \| Mortgage note payable, 5.720% interest rate, due in 2020
Fair Value of Financial Instruments
Senior Notes	36,085	0
Carrying Amount \| Mortgage note payable, 4.220% interest rate, due in 2022
Fair Value of Financial Instruments
Senior Notes	27,906	0
Carrying Amount \| Mortgage note payable, 4.800% interest rate, due in 2023
Fair Value of Financial Instruments
Senior Notes	25,501	0
Carrying Amount \| Mortgage note payable, 5.877% interest rate, due in 2021
Fair Value of Financial Instruments
Mortgage notes payable	13,620	13,841
Carrying Amount \| Mortgage note payable, 7.000% interest rate, due in 2019
Fair Value of Financial Instruments
Mortgage notes payable	8,391	8,778
Carrying Amount \| Mortgage note payable, 8.150% interest rate, due in 2021
Fair Value of Financial Instruments
Mortgage notes payable	4,111	5,218
Carrying Amount \| Mortgage note payable, 4.260% interest rate, due in 2020
Fair Value of Financial Instruments
Mortgage notes payable	3,193	0
Fair Value
Fair Value of Financial Instruments
Mortgage notes payable	1,164,606	675,601
Fair Value \| Senior unsecured notes, 3.750% interest rate, due in 2019
Fair Value of Financial Instruments
Senior Notes	354,993	354,078
Fair Value \| Senior unsecured notes, 5.875% interest rate, due in 2046
Fair Value of Financial Instruments
Senior Notes	320,416	292,268
Fair Value \| Senior unsecured notes, 4.000% interest rate, due in 2022
Fair Value of Financial Instruments
Senior Notes	302,655	0
Fair Value \| Mortgage note payable, 4.050% interest rate, due in 2030
Fair Value of Financial Instruments
Senior Notes	65,198	0
Fair Value \| Mortgage note payable, 5.720% interest rate, due in 2020
Fair Value of Financial Instruments
Senior Notes	36,332	0
Fair Value \| Mortgage note payable, 4.220% interest rate, due in 2022
Fair Value of Financial Instruments
Senior Notes	28,432	0
Fair Value \| Mortgage note payable, 4.800% interest rate, due in 2023
Fair Value of Financial Instruments
Senior Notes	25,904	0
Fair Value \| Mortgage note payable, 5.877% interest rate, due in 2021
Fair Value of Financial Instruments
Mortgage notes payable	14,565	14,492
Fair Value \| Mortgage note payable, 7.000% interest rate, due in 2019
Fair Value of Financial Instruments
Mortgage notes payable	8,555	9,188
Fair Value \| Mortgage note payable, 8.150% interest rate, due in 2021
Fair Value of Financial Instruments
Mortgage notes payable	4,340	5,575
Fair Value \| Mortgage note payable, 4.260% interest rate, due in 2020
Fair Value of Financial Instruments
Mortgage notes payable	$ 3,216	$ 0

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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+ References

No definition available.

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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://www.xbrl.org/2003/role/presentationRef
-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 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.16(a))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-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

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