4. RECEIVABLES

Receivables include current notes, amounts billed to customers, claims, other receivables, and unbilled revenue on long-term contracts, which consists of amounts recognized as sales but not billed. Substantially all amounts of unbilled receivables are expected to be billed and collected in the subsequent year. An immaterial amount of unbilled receivables are subject to retainage provisions. The amount of claims and unapproved change orders within our receivables balances are immaterial.

Credit risk is diversified due to the large number of entities comprising the Corporation’s customer base and their geographic dispersion. The Corporation is either a prime contractor or subcontractor to various agencies of the U.S. Government. Revenues derived directly and indirectly from government sources (primarily the U.S. Government) were 39% and 38% of total net sales in 2017 and 2016, respectively. Total receivables due primarily from the U.S Government were $208.4 million and $183.6 million as of December 31, 2017 and 2016, respectively. Government (primarily the U.S. Government) unbilled receivables, net of progress payments, were $89.3 million and $83.2 million as of December 31, 2017 and 2016, respectively.

The composition of receivables as of December 31 is as follows:



(In thousands)	2017			2016
Billed receivables:
Trade and other receivables	$	363,234		$	340,091
Less: Allowance for doubtful accounts	(7,486		)	(4,832		)
Net billed receivables	355,748			335,259
Unbilled receivables:
Recoverable costs and estimated earnings not billed	160,727			149,847
Less: Progress payments applied	(21,552		)	(22,044		)
Net unbilled receivables	139,175			127,803
Receivables, net	$	494,923		$	463,062