Employee Retirement Plans - Obligations and funded status (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Accumulated Benefit Obligations	$ 453.2	$ 490.0
Projected Benefit Obligations:
Beginning of year	490.0	459.6
Service cost	0.1	0.2	$ 0.4
Interest	18.3	19.6	20.8
Benefits paid	20.2	19.9
Actuarial loss	(35.6)	30.5
Other	0.6	0.0
End of year	453.2	490.0	459.6
Plans' Assets:
Beginning of year	488.0	442.5
Actual return on assets	(20.7)	62.9
Employer contributions	31.6	2.5
Benefits paid	(20.2)	(19.9)
End of year	478.7	488.0	$ 442.5
Consolidated Balance Sheet Components:
Net funded status	$ 25.5	$ (2.0)
Percent of projected benefit obligations funded	105.60%	99.60%
Other assets
Consolidated Balance Sheet Components:
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position	$ 39.4	$ 13.1
Accrued liabilities
Consolidated Balance Sheet Components:
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position	$ (13.9)	$ (15.1)