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, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets acquired and liabilities assumed
Multi-employer withdrawal liabilities			$ (60.1)				$ (56.2)			$ (60.1)	$ (56.2)
Goodwill			323.7				241.6			323.7	241.6	$ 27.4
Repayments of Span Alaska debt										81.9
Estimated future benefit payments
Mass withdrawal liability undiscounted payments			96.8							96.8
Customer relationships
Assets acquired and liabilities assumed
Expected useful lives	20 years
Span Alaska
Business acquisition
Percentage interest acquired	100.00%
Number of years in operation	38 years
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
Repayments of Span Alaska debt	81.9
Total consideration for membership interests	(117.0)
Condensed Consolidated Statements of Income and Comprehensive Income
Operating revenue										22.8
Operating Income										3.5
One-time acquisition related costs			0.2	$ 2.6	$ 0.1	$ 0.1				3.0
Span Alaska | Trade Names
Assets acquired and liabilities assumed
Intangibles – Trade name	27.3
Span Alaska | Customer relationships
Assets acquired and liabilities assumed
Intangibles – Customer relationships	$ 79.3
Span Alaska | Adjustments
Assets acquired and liabilities assumed
Total identifiable assets less liabilities				$ 0.4
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 liabilities												(65.5)
Capital lease obligations												(1.6)
Total identifiable assets less liabilities												(188.3)
Common shares		$ (29.4)										(29.4)
Goodwill												217.7
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							$ 1.5	$ 5.1	$ 12.4		$ 19.0
Estimated future benefit payments
Quarterly payments to ILA-PRSSA over an estimated period			$ 1.0							$ 1.0
Horizon | As previously Reported
Total cash consideration
Common shares												29.4
Assets acquired and liabilities assumed
Cash and cash equivalents												0.8
Accounts receivable												31.7
Other current assets												7.2
Deferred tax assets, net												45.6
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												(32.1)
Multi-employer withdrawal liabilities												(60.6)
Capital lease obligations												(1.6)
Total identifiable assets less liabilities												(184.8)
Common shares												(29.4)
Goodwill												214.2
Horizon's debt and warrants												(467.5)
Horizon | Adjustments
Assets acquired and liabilities assumed
Deferred tax assets, net												0.7
Accruals and other current liabilities												0.7
Multi-employer withdrawal liabilities												(4.9)
Total identifiable assets less liabilities												(3.5)
Goodwill												$ 3.5