Secured and unsecured senior debt Narratives (Details) $ in Thousands	Mar. 31, 2016 USD ($)
Debt Instrument [Line Items]
Effective rate (as a percent)	3.39%
Outstanding Balance	$ 4,119,144
Unsecured Bank Term Loan 2021 [Member]
Debt Instrument [Line Items]
Effective rate (as a percent)	1.74%
Outstanding Balance	$ 350,000
Construction Loans
Debt Instrument [Line Items]
Outstanding Balance	386,103
Secured Debt from Bank Maturing on 1 July 2016 [Member] \| Construction Loans \| San Francisco Bay Area
Debt Instrument [Line Items]
Outstanding Balance	$ 47,821

Amount of long-term debt before deduction of unamortized discount or premium. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, with initial maturities beyond one year or the normal operating cycle, if longer.

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 SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 22
-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 20
-Article 5

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 03
-Paragraph 16
-Article 9

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