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Contract Assets and Liabilities - Summary of assets and liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Contractors [Line Items]
Contract assets	$ 217,109	$ 160,281
Contract liabilities	(105,486)	(58,534)
Net contract assets (liabilities)	111,623	101,747
Changes in contract assets	16,443	$ 28,141	$ 17,857
Net Period Change [Member]
Contractors [Line Items]
Changes in contract assets	56,828
Changes in contract liabilities	(46,952)
Changes in net contract assets (liabilities)	$ 9,876

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The amount of increase(decrease)in net contract assets(liabilities) during the year.

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Amount, after allowance for credit loss, of right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time, classified as current.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable, classified as current.

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