Financial Assets and Liabilities - Level 3 financial instruments (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 45,856	$ 40,588
Purchases	0	0
Total Realized and Unrealized Gains / (Losses)	424	(345)
Net Transfers into (out of) Level 3	0	0
Settlement	0	0
Ending Balance	46,280	40,243
Change in Net Unrealized Gains / (Losses) on Investments still held at end of period	424	(345)
Equity investment
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	45,856	40,588
Purchases	0	0
Total Realized and Unrealized Gains / (Losses)	424	(345)
Net Transfers into (out of) Level 3	0	0
Settlement	0	0
Ending Balance	46,280	40,243
Change in Net Unrealized Gains / (Losses) on Investments still held at end of period	$ 424	$ (345)