Commitments and contingencies (Details) (USD $)	12 Months Ended
	Dec. 31, 2013 Tenant Lease property Parcel	Dec. 31, 2012	Dec. 31, 2011
Concentration Risk [Line Items]
Discretionary profit sharing contributions subject to statutory limitations	$ 1,600,000	$ 1,400,000	$ 1,300,000
FDIC insurance coverage	250,000
Largest aggregate notional amount in effect at any single point	250,000,000
Number of leases held	527
Number of client tenants	425
Number of properties leased	180
Remaining aggregate costs under contracts, under terms of leases	272,000,000
Letters of credit and performance obligations	13,900,000
Ownership interest in subsidiary (as a percent)	100.00%
Rent expense	11,400,000	10,600,000	10,200,000
Number of properties under terms of ground leases	29
Number of land development parcels under terms of ground leases	2
Term of operating lease obligation related to office leases	6 years
Number of ground leases related to operating property that expires in 2036	1
Net book value	8,900,000
Single Tenant | Lessee Concentration
Concentration Risk [Line Items]
Number of properties leased	79
Concentration risk, percentage	43.90%
Three Largest Tenants | Lessee Concentration | Annualized Base Rent
Concentration Risk [Line Items]
Number of largest tenants	3
Concentration risk, percentage	14.40%
First Largest Tenant | Lessee Concentration | Annualized Base Rent
Concentration Risk [Line Items]
Concentration risk, percentage	6.70%
Second Largest Tenant | Lessee Concentration | Annualized Base Rent
Concentration Risk [Line Items]
Concentration risk, percentage	3.90%
Third Largest Tenant | Lessee Concentration | Annualized Base Rent
Concentration Risk [Line Items]
Concentration risk, percentage	3.80%
Minimum
Concentration Risk [Line Items]
Expected period of payment obligation	1 year
Term of ground lease obligation	40 years
Maximum
Concentration Risk [Line Items]
Expected period of payment obligation	3 years
Term of ground lease obligation	100 years
Funding Investments
Concentration Risk [Line Items]
Fund commitment on certain investments	46,700,000
Project Development Agreements
Concentration Risk [Line Items]
Land available for development	$ 18,800,000
Area of land that is under construction commitment obligations	300,000