Commitments and Contingencies - Summary of accrued short- and long-term insurance liabilities (Details) - Insurance Claims [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accrued short- and long-term insurance liabilities
Balance at beginning of period	$ 45,363	$ 42,584	$ 36,967
Net increases in reserves	31,193	22,938	25,139
Net payments made	(22,517)	(20,159)	(19,522)
Balance at end of period	$ 54,039	$ 45,363	$ 42,584