Retirement and Other Postretirement Benefit Plans - Schedule of Pension Assets Measured at Fair Value (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value measurements	$ 201.8	$ 171.4
Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value measurements	56.3	53.8
Diversified Growth Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value measurements	48.0	41.2
Liability Driven Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value measurements	38.7	34.4
Other Insurance Product Line
Defined Benefit Plan Disclosure [Line Items]
Fair value measurements	4.2	3.6
Index Linked Gilts
Defined Benefit Plan Disclosure [Line Items]
Fair value measurements	52.1	36.1
Diversified Investment, Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value measurements	2.3	2.1
Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value measurements	0.2	0.2
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value measurements	0.2	0.2
Level 1 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value measurements	0.2	0.2
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value measurements	195.1	165.5
Level 2 | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value measurements	56.3	53.8
Level 2 | Diversified Growth Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value measurements	48.0	41.2
Level 2 | Liability Driven Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value measurements	38.7	34.4
Level 2 | Index Linked Gilts
Defined Benefit Plan Disclosure [Line Items]
Fair value measurements	52.1	36.1
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value measurements	6.5	5.7	$ 5.6
Actual return on plan assets	0.3	0.3
Purchases, sales and settlements	(0.3)
Changes due to exchange rates	0.8	(0.2)
Level 3 | Other Insurance Product Line
Defined Benefit Plan Disclosure [Line Items]
Fair value measurements	4.2	3.6	3.5
Actual return on plan assets	0.2	0.2
Purchases, sales and settlements	(0.1)
Changes due to exchange rates	0.5	(0.1)
Level 3 | Diversified Investment, Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value measurements	2.3	2.1	$ 2.1
Actual return on plan assets	0.1	0.1
Purchases, sales and settlements	(0.2)
Changes due to exchange rates	$ 0.3	$ (0.1)