Investment in equity method investee - Company's Distribution and Earnings in Equity Method Investment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Schedule of Equity Method Investments [Line Items]
Distribution received from CP&P			$ (4,500)	$ (1,500)
Share of earnings in CP&P	$ 3,126	$ 3,402	5,925	4,969
CP&P Joint Venture
Schedule of Equity Method Investments [Line Items]
Distribution received from CP&P	(2,900)	0	(4,500)	(1,500)
Share of earnings in CP&P	3,146	3,420	5,961	5,005
Amortization of excess fair value of investment	$ (18)	$ (18)	$ (36)	$ (36)