Goodwill - Changes in Carrying Amount of Definite-Lived Intangible Assets Excluding Goodwill (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Definite-lived intangible assets, gross carrying value		$ 93.8		$ 1.9
Accumulated amortization		5.5	[1]	0.5
Definite-lived intangible assets, net carrying value		88.3		1.4
Intangible assets (excluding goodwill), gross carrying value		157.3	[1]	10.7
Intangible assets, net (excluding goodwill)		$ 151.8	[1]	10.2
Definite-lived intangible asset, useful life		12 years
Accumulated expense		$ 4.8
Estimated amortization of intangibles, 2018		8.0
Estimated amortization of intangibles, 2019		8.0
Estimated amortization of intangibles, 2020		8.0
Estimated amortization of intangibles, 2021		8.0
Estimated amortization of intangibles, 2022		7.7
Estimated amortization of intangibles, thereafter		48.6
Customer Relationships
Finite-Lived Intangible Assets [Line Items]
Definite-lived intangible assets, gross carrying value	[2]	90.9		0.8
Accumulated amortization	[2]	4.6		0.1
Definite-lived intangible assets, net carrying value	[2]	$ 86.3		0.7
Definite-lived intangible asset, useful life		12 years
Other Intangible Assets
Finite-Lived Intangible Assets [Line Items]
Definite-lived intangible assets, gross carrying value	[2]	$ 2.9		1.1
Accumulated amortization	[2]	0.9		0.4
Definite-lived intangible assets, net carrying value	[2]	$ 2.0		$ 0.7
Definite-lived intangible asset, useful life		6 years

[1] Amounts at December 31, 2017 include intangible assets acquired in the TBEI acquisition - see Note 2 – Acquisitions.

[2] Average useful life of customer relationships and other definite-lived intangible assets are estimated to be approximately 12 years and 6 years, respectively. The average useful life across all definite-lived intangible assets is estimated to be approximately 12 years.