Derivative financial instruments - Effects on Income Statement (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Beginning balance				$ 839,841				$ 599,549				$ 170,603	$ 839,841	$ 599,549	$ 170,603
Total other comprehensive (loss) income													(20,984)	168	(107)
Ending balance	$ 809,651				$ 839,841				$ 599,549				809,651	839,841	599,549
Total amount expected to be reclassified from AOCI to interest expense during next twelve months													6,200
Interest rate swaps | Interest expense
Derivative Instruments, Gain (Loss) [Line Items]
Effects on Income Statement													2,715	17,093	0
Cash Flow Hedge
Derivative Instruments, Gain (Loss) [Line Items]
Beginning balance				0									0
Change in fair value	(3,907)	$ 4,912	$ 14,648	5,834
Reclassification from AOCI to interest expense	(1,159)	(324)	136	24
Total other comprehensive (loss) income		$ 4,588	$ 14,784	5,858
Ending balance	$ 20,164				0								20,164	0
Cash Flow Hedges
Derivative Instruments, Gain (Loss) [Line Items]
Beginning balance				$ 0				0				0	$ 0	0	0
Change in fair value					0	$ 0	$ 0	0	0	$ 0	$ 0	0
Reclassification from AOCI to interest expense					0	0	0	0	0	0	0	0
Total other comprehensive (loss) income						$ 0	$ 0	$ 0		$ 0	$ 0	$ 0
Ending balance					$ 0				$ 0					$ 0	$ 0