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.

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))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: 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))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: 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

Name:	us-gaap_LineOfCredit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Unsecured Debt - Summary (Details) - USD ($) $ in Thousands	12 Months Ended
Unsecured Debt - Summary (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Nov. 30, 2016	Oct. 31, 2016
Debt Instrument [Line Items]
Unsecured debt, net	$ 3,501,709	$ 3,246,779
Lines of credit	179,000	125,000
Total unsecured debt	3,680,709	3,371,779
Unsecured bonds, private placement [Member]
Debt Instrument [Line Items]
Unsecured debt, net	$ 274,427	314,190
Debt instruments, maturity period	3 years 1 month 15 days
Variable rate term loan [Member]
Debt Instrument [Line Items]
Unsecured debt, net		$ 100,000	$ 350,000	$ 225,000
Weighted average interest rate	2.50%	2.30%
Debt instruments, maturity period	49 months 15 days
Variable rate term loan [Member] \| Carrying Value [Member]
Debt Instrument [Line Items]
Unsecured debt, net	$ 348,545	$ 98,189
Bonds public offering - fixed rate [Member]
Debt Instrument [Line Items]
Unsecured debt, net	$ 2,878,737	$ 2,834,400
Weighted average interest rate	3.70%	3.60%
Debt instruments, maturity period	6 years 4 months 15 days
Unsecured Line of Credit [Member]
Debt Instrument [Line Items]
Lines of credit	$ 179,000	$ 125,000
Weighted average interest rate	2.30%	1.80%