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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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Operating income	$ 9,732	$ 12,115	$ 11,184	$ 4,720	$ 9,410	$ 14,373	$ 12,149	$ 6,679	$ 37,751	$ 42,611	$ 26,176
Net income attributable to Sterling common stockholders	$ 22,301	$ 7,957	$ 7,828	$ 1,815	$ 5,607	$ 8,917	$ 8,174	$ 2,489	39,901	25,187	11,617
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Revenues									205,615	193,677	181,589
Operating income									6,372	8,819	9,069
Net income attributable to Sterling common stockholders									$ 3,196	$ 4,415	$ 4,531