Goodwill and Intangible Assets (Tables)

12 Months Ended

Dec. 31, 2018

Goodwill and Intangible Assets Disclosure [Abstract]

Schedule of Goodwill

The table below provides a summary of our change in carrying amount of goodwill (in millions) for the year ended December 31, 2018, by assigned reporting unit. For the year ended December 31, 2017, there were no changes to the carrying amounts of goodwill.

	Texas			Oklahoma		Corporate		Totals
Year Ended December 31, 2018
Balance, beginning of period	$	232.0		$	190.3	$	1,119.9	$	1,542.2
Impairment	(232.0		)	—		—		(232.0		)
Balance, end of period	$	—		$	190.3	$	1,119.9	$	1,310.2

Summary of Changes in Carrying Value

The following table represents our change in carrying value of intangible assets for the periods stated (in millions):

	Gross Carrying Amount		Accumulated Amortization			Net Carrying Amount
Year Ended December 31, 2018
Customer relationships, beginning of period	$	1,795.8	$	(298.7	)	$	1,497.1
Amortization expense	—		(123.5		)	(123.5		)
Customer relationships, end of period	$	1,795.8	$	(422.2	)	$	1,373.6
Year Ended December 31, 2017
Customer relationships, beginning of period	$	1,795.8	$	(171.6	)	$	1,624.2
Amortization expense	—		(127.1		)	(127.1		)
Customer relationships, end of period	$	1,795.8	$	(298.7	)	$	1,497.1
Year Ended December 31, 2016
Customer relationships, beginning of period	$	744.5	$	(54.6	)	$	689.9
Acquisitions	1,051.3		—			1,051.3
Amortization expense	—		(117.0		)	(117.0		)
Customer relationships, end of period	$	1,795.8	$	(171.6	)	$	1,624.2

Schedule of Amortization Expense

The following table summarizes our estimated aggregate amortization expense for the next five years and thereafter (in millions):

2019	$	123.7
2020	123.7
2021	123.7
2022	123.7
2023	123.6
Thereafter	755.2
Total	$	1,373.6