Intangibles, Net
 12 Months Ended
Dec. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract]  
Intangibles, Net
8.
Intangibles, Net
 
As of December 31, 2015
 
Remaining
Useful
Life (Years)
 
Gross
 
Accumulated
Amortization
 
Net
Customer relationships
3-15
 
$
30,426

 
$
10,204

 
$
20,222

Other
4-6
 
5,121

 
3,230

 
1,891

Intangible assets, net
 
 
$
35,547

 
$
13,434

 
$
22,113

 
As of December 31, 2014
 
Remaining
Useful
Life (Years)
 
Gross
 
Accumulated
Amortization
 
Net
Customer relationships
5-16
 
$
31,961

 
$
6,853

 
$
25,108

Other
5-7
 
4,671

 
2,153

 
2,518

Intangible assets, net
 
 
$
36,632

 
$
9,006

 
$
27,626



The Partnership recorded amortization expense related to intangible assets of $4.4 million, $3.2 million and $2.7 million during the years ended December 31, 2015, 2014 and 2013, respectively. The amortization of intangible assets is recorded in depreciation and amortization expense.
During the year ended December 31, 2014, the Partnership acquired intangible assets of $19.2 million (consisting of $18.2 million of customer relationships and $1.0 million of other intangibles). During the year ended December 31, 2013, the Predecessor acquired intangible assets of $0.5 million consisting of customer relationships. See Note 2.
Amortization of intangible assets is calculated by the sum-of-the-years’-digits method over the periods of expected benefit. The Partnership believes the sum-of-the-years’-digits method of amortization properly reflects the timing of the recognition of the economic benefits realized from its intangible assets. The estimated future annual amortization expense of intangible assets for the years ending December 31, 2016, 2017, 2018, 2019 and 2020 is $4.0 million, $3.4 million, $2.8 million, $2.4 million and $2.0 million, respectively. As acquisitions and dispositions occur in the future, these amounts may vary.