Derivative financial instruments - Effects on Comprehensive Income Statement (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 772,681	$ 809,651	$ 878,046	$ 839,841	$ 809,651	$ 839,841
Total other comprehensive income (loss)	3,078	(15,103)	(14,809)	(6,009)	(12,025)	(20,818)
Ending balance	710,558	772,681	863,891	878,046	710,558	863,891
Total amount expected to be reclassified from AOCI to interest expense during next twelve months					11,700
Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		20,164		0	20,164	0
Change in fair value	0	16,956	14,648	5,834
Reclassification from AOCI to interest expense	(2,926)	(1,908)	136	24
Total other comprehensive income (loss)	(2,926)	$ 15,048	14,784	$ 5,858
Ending balance	$ 32,286		$ 20,642		$ 32,286	$ 20,642