Investment in equity method investee - Company's Distribution and Earnings in Equity Method Investment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Schedule of Equity Method Investments [Line Items]
Distribution received from CP&P	$ (1,600)	$ (1,500)
Share of earnings in CP&P	2,799	1,567
CP&P Joint Venture
Schedule of Equity Method Investments [Line Items]
Distribution received from CP&P	(1,600)	(1,500)
Share of earnings in CP&P	2,816	1,585
Amortization of excess fair value of investment	$ (18)	$ (18)