Note 4. Inventories

Our inventory amounts by product type were as follows at the dates indicated:

	December 31,
	2017		2016
NGLs	$	917.4	$	1,156.1
Petrochemicals and refined products		161.5		220.7
Crude oil		516.3		360.0
Natural gas		14.6		33.7
Total	$	1,609.8	$	1,770.5

In those instances where we take ownership of inventory volumes through percent-of-liquids contracts and similar arrangements (as opposed to outright purchases from third parties for cash), these volumes are valued at market-based prices during the month in which they are acquired.

The following table presents our total cost of sales amounts and lower of cost or net realizable value adjustments for the periods indicated:

	For the Year Ended December 31,
	2017		2016		2015
Cost of sales (1)	$	21,487.0	$	15,710.9	$	19,612.9
Lower of cost or net realizable value adjustments within cost of sales		9.1		11.5		19.8

(1) Cost of sales is a component of “Operating costs and expenses,” as presented on our Statements of Consolidated Operations. Fluctuations in these amounts are primarily due to changes in energy commodity prices and sales volumes associated with our marketing activities.

Due to fluctuating commodity prices, we recognize lower of cost or net realizable value adjustments when the carrying value of our available-for-sale inventories exceeds their net realizable value. These non-cash charges are a component of cost of sales in the period they are recognized. To the extent our commodity hedging strategies address inventory-related price risks and are successful, these inventory valuation adjustments are mitigated or offset. See Note 14 for a description of our commodity hedging activities.