Revenue Recognition - Contract Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018
Revenue from Contract with Customer [Abstract]
Customer receivables	$ 744,000	$ 748,000
Contract liabilities	392,000	353,000
Accounts receivable, gross	753	751
Allowance for doubtful accounts receivable	$ 9	$ 3

For unclassified balance sheet, amounts due from customers or clients for goods or services that have been delivered or sold in the normal course of business.

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For an unclassified balance sheet, the amount due from customers or clients for goods or services that have been delivered or sold in the normal course of business, reduced to their estimated net realizable fair value by an allowance established by the entity of the amount it deems uncertain of collection.

+ References

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- Definition

For an unclassified balance sheet, a valuation allowance for receivables due a company that are expected to be uncollectible.

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- Definition

Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable.

+ References

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