INTANGIBLE ASSETS

12 Months Ended

Dec. 31, 2016

Intangible Assets, Net (Excluding Goodwill) [Abstract]

INTANGIBLE ASSETS

14. INTANGIBLE ASSETS

The following table provides the estimated useful life, gross carrying value, accumulated amortization and net carrying value for each of our major classes of intangible assets:

			December 31, 2016						December 31, 2015
		Estimated			Accumulated						Accumulated
		Useful Life	Gross		Amortization		Net		Gross		Amortization		Net
	(in millions)
Natural gas supply opportunities		15 – 30 years	$	324.1	$	(94.4)	$	229.7	$	324.1	$	(81.8)	$	242.3
Other intangible assets		3 – 25 years		94.6		(65.8)		28.8		102.2		(64.5)		37.7
Total intangible assets, net			$	418.7	$	(160.2)	$	258.5	$	426.3	$	(146.3)	$	280.0

Intangible assets primarily include natural gas supply opportunities, which are derived from growth opportunities present in the Barnett Shale producing zone of North Texas and the Granite Wash reservoir of the Anadarko basin in western Oklahoma and the Texas Panhandle. These natural gas supply opportunities primarily consist of dedicated acreage, whereby any prospective producers commencing drilling in areas served by our assets would be required to connect to our systems.

Other intangible assets primarily include software, customer contracts and contributions in aid of construction, or CIACs. These other intangible assets have estimated useful lives that range as short as three years for software to as long as 25 years for CIACs.

For the years ended December 31, 2016, 2015 and 2014, our amortization expense related to intangible assets totaled $23.1 million, $22.2 million and $15.5 million, respectively. The following table presents our forecast of amortization expense associated with existing intangible assets for the years indicated as follows in millions:

2017		2018		2019		2020		2021
$	18.5	$	13.8	$	13.8	$	13.8	$	13.8