Leases - Components of leasing revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Leases [Abstract]
Leasing revenue - fixed payments	$ 159,356		$ 482,194
Leasing revenue - fixed payments		$ 164,351		$ 494,096
Leasing revenue - variable payments	54,904		154,096
Leasing revenue - variable payments		59,900		165,281
Total leasing revenue	$ 214,260		$ 636,290
Total leasing revenue		$ 224,251		$ 659,377