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Consolidated 50% Owned Subsidiaries, including Variable Interest Entities ("VIE") - Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	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, 2018	Dec. 31, 2017	Dec. 31, 2016
Variable Interest Entity [Line Items]
Operating income									$ 42,611	$ 26,176	$ (4,729)
Net income attributable to Sterling common stockholders	$ 5,607	$ 8,915	$ 8,176	$ 2,489	$ 3,080	$ 7,132	$ 3,662	$ (2,257)	25,187	11,617	(9,238)
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Revenues									193,677	181,589	156,202
Operating income									8,819	9,069	6,005
Net income attributable to Sterling common stockholders									$ 4,415	$ 4,531	$ 2,993