BUSINESS COMBINATION - CONSIDERATION PAID (Details) - USD ($) $ in Millions			3 Months Ended				12 Months Ended
	Aug. 04, 2016	May 29, 2015	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets acquired and liabilities assumed
Goodwill			$ 323.7				$ 323.7	$ 323.7	$ 241.6
Repayments of Span Alaska debt								81.9
Estimated future benefit payments
Mass withdrawal liability undiscounted payments							92.6
Quarterly payments to ILA-PRSSA over an estimated period							1.0
Span Alaska
Business acquisition
Percentage interest acquired	100.00%
Total cash consideration
Membership interests	$ 117.0
Total debt assumed	81.9
Total payments	198.9
Assets acquired and liabilities assumed
Cash and cash equivalents	4.4
Accounts receivable	11.1
Prepaid and other current assets	0.9
Property and equipment	8.1
Intangibles – Customer relationships	79.3
Intangibles – Trade name	27.3
Other long-term assets	0.1
Accounts payable	(3.3)
Accruals and other current liabilities	(6.4)
Capital lease obligations	(1.2)
Span Alaska's debt	(81.9)
Total identifiable assets less liabilities	38.4
Goodwill	$ 78.6
Condensed Consolidated Statements of Income and Comprehensive Income
Operating revenue							59.1	22.8
Operating Income							$ 12.8	3.5
One-time acquisition related costs								3.0
Horizon
Total cash consideration
Common shares		$ 29.4							29.4
Warrants		37.1
Total debt assumed		428.9
Total payments		495.4
Assets acquired and liabilities assumed
Cash and cash equivalents									0.8
Accounts receivable									31.7
Other current assets									7.2
Deferred tax assets, net									46.3
Property and equipment									170.4
Intangibles – Customer relationships									140.0
Other long-term assets									4.1
Accounts payable									(22.8)
Accruals and other current liabilities									(31.4)
Multi-employer withdrawal liability									65.5
Capital lease obligations									(1.6)
Total identifiable assets less liabilities									(188.3)
Goodwill									217.7
Common shares		$ (29.4)							(29.4)
Horizon's debt and warrants									(467.5)
Condensed Consolidated Statements of Income and Comprehensive Income
Operating revenue								277.6
Operating Income								$ 20.0
One-time acquisition related costs			$ 0.2	$ 2.6	$ 0.1	$ 0.1			$ 19.0