Subsequent events (Details) $ / shares in Units, ft² in Millions				3 Months Ended								12 Months Ended
	Feb. 12, 2020 USD ($) ft² a property numberOfHotelRooms numberOfRestaurants $ / ft²	Feb. 10, 2020 USD ($)	Feb. 04, 2020 $ / shares	Dec. 31, 2019 USD ($) ft² a $ / shares	Sep. 30, 2019 $ / shares	Jun. 30, 2019 $ / shares	Mar. 31, 2019 $ / shares	Dec. 31, 2018 $ / shares	Sep. 30, 2018 $ / shares	Jun. 30, 2018 $ / shares	Mar. 31, 2018 $ / shares	Dec. 31, 2019 USD ($) ft² a
Subsequent Event [Line Items]
Dividends declared (in dollars per share) | $ / shares				$ 0.21	$ 0.21	$ 0.21	$ 0.21	$ 0.21	$ 0.21	$ 0.21	$ 0.21
Rentable square feet | ft²				16.0								16.0
Area of land | a				11.7								11.7
Unsecured 500 Million 2018 Line Of Credit | Unsecured Debt
Subsequent Event [Line Items]
Face amount of facility				$ 500,000,000								$ 500,000,000
Line of Credit | Unsecured 500 Million 2018 Line Of Credit | Unsecured Debt
Subsequent Event [Line Items]
Amount of line of credit				$ 500,000,000								$ 500,000,000
LIBOR | Unsecured 500 Million 2018 Line Of Credit | Unsecured Debt
Subsequent Event [Line Items]
Spread on variable rate												0.90%
Subsequent Event
Subsequent Event [Line Items]
Dividends declared (in dollars per share) | $ / shares			$ 0.21
Face amount of facility		$ 150,000,000
Subsequent Event | Dallas Galleria Office Towers
Subsequent Event [Line Items]
Rentable square feet | ft²	1.4
Number of office towers | property	3
Area of land | a	1.9
Area of mixed use development | ft²	3.7
Dining establishments | numberOfRestaurants	30
Area of retail space | ft²	1.5
Number of hotel rooms | numberOfHotelRooms	448
Asset contractual purchase price	$ 396,000,000
Price per square foot | $ / ft²	275
Subsequent Event | Line of Credit | Unsecured 500 Million 2018 Line Of Credit | Unsecured Debt
Subsequent Event [Line Items]
Amount of line of credit	$ 500,000,000
Subsequent Event | LIBOR
Subsequent Event [Line Items]
Spread on variable rate		0.95%