Employee Retirement Plans - Estimated fair value of plan assets and target asset allocations (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	100.00%
Actual Allocation	100.00%
Fair Value Measurement	$ 478.7	$ 488.0	$ 442.5
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair Value Measurement	14.0	7.3
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair Value Measurement	458.0	480.7
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair Value Measurement	$ 6.7	0.0
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	0.00%
Actual Allocation	3.00%
Fair Value Measurement	$ 14.0	7.3
Cash and cash equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair Value Measurement	14.0	7.3
Cash and cash equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair Value Measurement	0.0	0.0
Cash and cash equivalents | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair Value Measurement	0.0	0.0
US Treasury and Government [Member] | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair Value Measurement	$ 77.1
Liability hedging portfolio
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	90.00%
Actual Allocation	82.00%
Growth portfolio
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	10.00%
Actual Allocation	15.00%
Fixed Income - Collateralized mortgage backed
Defined Benefit Plan Disclosure [Line Items]
Fair Value Measurement		292.8
Fixed Income - Collateralized mortgage backed | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair Value Measurement		0.0
Fixed Income - Collateralized mortgage backed | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair Value Measurement	$ 309.1	292.8
Fixed Income - Collateralized mortgage backed | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair Value Measurement	6.7	0.0
Collateralized Mortgage Backed Securities [Member] | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair Value Measurement	0.0
Collateralized Mortgage Backed Securities [Member] | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair Value Measurement	2.1
Collateralized Mortgage Backed Securities [Member] | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair Value Measurement	0.0
Equity common trust funds
Defined Benefit Plan Disclosure [Line Items]
Fair Value Measurement	69.7	187.9
Equity common trust funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair Value Measurement	0.0	0.0
Equity common trust funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair Value Measurement	69.7	187.9
Equity common trust funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair Value Measurement	$ 0.0	$ 0.0