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Asset Retirement Obligations (Notes)
 12 Months Ended
Dec. 31, 2017
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligations
ASSET RETIREMENT OBLIGATIONS
These accrued obligations relate to underground storage caverns, offshore platforms, pipelines, and gas transmission facilities. At the end of the useful life of each respective asset, we are legally obligated to plug storage caverns and remove any related surface equipment, to dismantle offshore platforms, to cap certain gathering pipelines at the wellhead connection and remove any related surface equipment, and to remove certain components of gas transmission facilities from the ground.
During 2017 and 2016, our overall asset retirement obligation changed as follows (in thousands): 
 
 
2017
 
2016
Beginning balance
 
$
275,452

 
$
323,026

Accretion (1)
 
104,659

 
35,740

New obligations
 
28,447

 
7,995

Changes in estimates of existing obligations (2)
 
(38,470
)
 
(85,514
)
Property dispositions/obligations settled
 
(6,132
)
 
(5,795
)
Ending balance
 
$
363,956

 
$
275,452


(1)
The increase in accretion for 2017 includes the cumulative effect of accretion adjustment associated with new AROs identified in our historical land agreements of $87 million that are not a component of new obligations.
(2)
Changes in estimates of existing obligations are primarily due to the annual review process, which considers various factors including inflation rate, current estimates for removal cost, discount rates, and the estimated remaining life of assets. The changes in estimates of existing obligations reflect a decrease of $38 million and $86 million for 2017 and 2016, respectively. The decrease in 2017 is primarily due to a decrease in current estimates for offshore removal costs. The decrease in 2016 is due primarily to revisions in the estimated remaining life of assets and the current estimate to the inflation rate.

We are entitled to collect in rates the amounts necessary to fund our ARO. All funds received for such retirements are deposited into an external trust account dedicated to funding our ARO. Under our current rate settlement our annual funding obligation is approximately $36.4 million, with installments to be deposited monthly (See Note 4).