	December 31, 2020		December 31, 2019
Deferred revenue	$	168.5	$	172.0
Asset retirement obligations		68.3		66.3
Operating lease liabilities		46.2		47.2
Other liabilities		26.1		20.1
Total long-term liabilities	$	309.1	$	305.6

Our ARO primarily relate to certain gas gathering pipelines and processing facilities and NGL pipelines. The changes in our ARO are as follows:

	2020			2019
Beginning of period	$	66.3		$	55.5
Additions (1)		—			11.8
Change in cash flow estimate		(1.8	)		(5.1	)
Accretion expense		3.6			4.7
Retirement of ARO		0.2			(0.6	)
End of period	$	68.3		$	66.3

Deferred revenue for the years ended December 31, 2020 and 2019, was $168.5 million and $172.0 million, respectively, which includes $129.0 million of payments received from Vitol Americas Corp. (“Vitol”) (formerly known as Noble Americas Corp.), a subsidiary of Vitol US Holding Co. The payments were received in 2016, 2017, and 2018 as part of an agreement (the “Splitter Agreement”) related to the construction and operation of a crude oil and condensate splitter. In December 2018, Vitol elected to terminate the Splitter Agreement. The Splitter Agreement provides that the first three annual payments are ours if Vitol elects to terminate, which Vitol disputes. The timing of revenue recognition related to the Splitter Agreement deferred revenue is dependent upon resolution of the dispute with Vitol.

Deferred revenue also includes nonmonetary consideration received in a 2015 amendment (the “gas contract amendment”) to a gas gathering and processing agreement. We measured the estimated fair value of the gathering assets transferred to us using significant other observable inputs representative of a Level 2 fair value measurement. In December 2017, we received monetary consideration to further amend the terms of the gas gathering and processing agreement. The deferred revenue related to these amendments is being recognized on a straight-line basis through the end of the agreement’s term in 2035.

Deferred revenue also includes consideration received for other construction activities of facilities connected to our systems. The deferred revenue related to these other construction activities is being recognized over the periods that future performance will be provided, which extend through 2023.

For the years ended December 31, 2020, 2019 and 2018, we recognized approximately $3.8 million, $3.9 million and $3.9 million of revenue for these transactions, respectively.

	December 31, 2020		December 31, 2019
Splitter agreement	$	129.0	$	129.0
Gas contract amendment		37.3		39.8
Other deferred revenue		2.2		3.2
Total deferred revenue	$	168.5	$	172.0

	2020			2019
Balance at beginning of period	$	172.0		$	175.5
Additions		0.3			0.4
Revenue recognized		(3.8	)		(3.9	)
Balance at end of period	$	168.5		$	172.0