Employee Retirement Plans - Obligations and funded status - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Accumulated Benefit Obligations	$ 490.0	$ 459.6
Projected Benefit Obligations:
Beginning of year	459.6	445.3
Service cost	0.2	0.4	$ 0.5
Interest	19.6	20.8	20.0
Benefits paid	19.9	18.1
Actuarial loss	30.5	11.2
End of year	490.0	459.6	445.3
Plans' Assets:
Beginning of year	442.5	422.8
Actual return on assets	62.9	33.1
Employer contributions	2.5	4.7
Benefits paid	(19.9)	(18.1)
End of year	488.0	442.5	$ 422.8
Consolidated Balance Sheet Components:
Net funded status	$ (2.0)	$ (17.1)
Percent of projected benefit obligations funded	99.60%	96.30%
Other assets
Consolidated Balance Sheet Components:
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position	$ 13.1	$ 5.1
Accrued liabilities
Consolidated Balance Sheet Components:
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position	$ (15.1)	$ (22.2)