Significant Accounting Policies Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Property, Plant and Equipment

The components of property and equipment are as follows (in millions):

	Year Ended December 31,
	2017			2016
Transmission assets	$	1,338.7		$	1,191.7
Gathering systems	4,040.9			3,530.9
Gas processing plants	3,401.8			3,163.0
Other property and equipment	157.8			149.5
Construction in process	180.8			345.7
Property and equipment	$	9,120.0		$	8,380.8
Accumulated depreciation	(2,533.0		)	(2,124.1		)
Property and equipment, net of accumulated depreciation	$	6,587.0		$	6,256.7

Schedule Of Property Plant And Equipment Useful Lives

Depreciation is calculated using the straight-line method based on the estimated useful life of each asset, as follows:

	Useful Lives
Transmission assets	20 - 25 years
Gathering systems	20 - 25 years
Gas processing plants	20 - 25 years
Other property and equipment	3 - 15 years