Secured and unsecured senior debt Narratives (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Debt Instrument [Line Items]
Repayments of Unsecured Debt				$ 25,000	$ 125,000
Effective rate (as a percent)	[1]	2.97%		2.97%
Loss on early extinguishment of debt		$ 0	$ 525	$ 189	$ 525
Unsecured Bank Term Loan 2021 [Member]
Debt Instrument [Line Items]
Effective rate (as a percent)	[1]	1.52%		1.52%
Outstanding Balance		$ 350,000		$ 350,000
Construction Loans [Member]
Debt Instrument [Line Items]
Outstanding Balance		219,603		219,603
Secured Debt from Bank Maturing on 1 July 2016 [Member] | Construction Loans [Member] | San Franciso Bay Area [Member]
Debt Instrument [Line Items]
Outstanding Balance		$ 47,385		$ 47,385

[1] Represents the weighted-average interest rate as of the end of the period plus the impact of debt premiums/discounts and our interest rate swap agreements. The weighted-average interest rate excludes bank fees and amortization of loan fees.