Other Assets

Other assets consist of the following (in thousands):



	December 31,
	2018			2017
Costs to obtain contracts	$	219,307		$	157,970
Accumulated amortization of costs to obtain contracts	(24,992		)	(16,485		)
Unbilled receivables	81,703			51,710
Operating lease right-of-use assets	20,257			25,465
Other assets	39,410			27,804
Total	$	335,685		$	246,464

The Company recorded amortization of costs to obtain contracts of $8.6 million and $6.5 million for the years ended December 31, 2018 and 2017, respectively, in the sales and marketing expense.

The majority of unbilled receivables arise from fixed price escalators included in our long-term Customer Agreements. The escalator is included in calculating the total estimated transaction value for an individual Customer Agreement. The total estimated transaction value is then recognized over the term of the Customer Agreement. The amount of unbilled receivables increases while cumulative billings for an individual Customer Agreement are less than the cumulative revenue recognized for that Customer Agreement. Conversely, the amount of unbilled receivables decreases when the actual cumulative billings becomes higher than the cumulative revenue recognized. At the end of the initial term of a Customer Agreement, the cumulative amounts recognized as revenue and billed to date are the same, therefore the unbilled receivable balance for an individual Customer Agreement will be zero.