Derivative financial instruments - Effects on Comprehensive Loss Statement (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 710,558	$ 772,681	$ 809,651	$ 863,891	$ 878,046	$ 839,841	$ 809,651	$ 839,841
Total other comprehensive income (loss)	3,058	3,078	(15,103)	(4,594)	(14,809)	(6,009)	(8,967)	(25,412)
Ending balance	636,937	710,558	772,681	824,301	863,891	878,046	636,937	824,301
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	0	16,956	4,912	14,648	5,834
Reclassification from AOCI to interest expense	(2,958)	(2,926)	(1,908)	(324)	136	24
Total other comprehensive income (loss)	(2,958)	$ (2,926)	$ 15,048	4,588	$ 14,784	$ 5,858
Ending balance	$ 29,328			$ 25,230			$ 29,328	$ 25,230