The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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Goodwill and Intangible Assets Goodwill (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Dec. 31, 2017
Goodwill [Line Items]
Goodwill, Acquired During Period			$ 0
Goodwill	$ 452,700,000	$ 452,700,000	452,700,000	$ 452,700,000	$ 452,700,000	$ 452,700,000
Goodwill, Impaired, Accumulated Impairment Loss	0		0		$ 0
Amortization of intangible assets	$ 18,000,000	$ 15,500,000	$ 36,000,000	$ 31,000,000
Weighted Average [Member]
Goodwill [Line Items]
Finite-Lived Intangible Assets, Remaining Amortization Period			4 years 4 months 25 days
Trade name [Member]
Goodwill [Line Items]
Finite-Lived Intangible Asset, Useful Life				10 years	2 years