Pension Plans and Other Postretirement Benefits - Net Periodic Benefit Costs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Net periodic benefit cost
Pension settlement loss				$ 0.7
Defined Benefit Pension Plans
Net periodic benefit cost
Service cost	$ 4.2	$ 3.1	$ 8.6	6.3
Interest cost	0.8	1.2	1.6	2.5
Expected return on plan assets	(0.3)	(0.5)	(0.6)	(1.0)
Amortization of prior service cost (credit)	(0.3)	(0.3)	(0.6)	(0.5)
Amortization of net (gain) loss	1.2	0.8	2.3	1.5
Pension settlement loss				0.7
Net periodic benefit cost	$ 5.6	$ 4.3	$ 11.3	$ 9.5
Interest cost, Statements of Operations location	us-gaap:OtherNonoperatingIncomeExpense	us-gaap:OtherNonoperatingIncomeExpense	us-gaap:OtherNonoperatingIncomeExpense	us-gaap:OtherNonoperatingIncomeExpense
Expected return, Statements of Operations location	us-gaap:OtherNonoperatingIncomeExpense	us-gaap:OtherNonoperatingIncomeExpense	us-gaap:OtherNonoperatingIncomeExpense	us-gaap:OtherNonoperatingIncomeExpense
Amortization of prior service credit, Statements of Operations location	us-gaap:OtherNonoperatingIncomeExpense	us-gaap:OtherNonoperatingIncomeExpense	us-gaap:OtherNonoperatingIncomeExpense	us-gaap:OtherNonoperatingIncomeExpense
Amortization of gain (loss), Statements of Operations location	us-gaap:OtherNonoperatingIncomeExpense	us-gaap:OtherNonoperatingIncomeExpense	us-gaap:OtherNonoperatingIncomeExpense	us-gaap:OtherNonoperatingIncomeExpense
Net settlement and curtailment loss, Statements of Operations location	us-gaap:OtherNonoperatingIncomeExpense	us-gaap:OtherNonoperatingIncomeExpense	us-gaap:OtherNonoperatingIncomeExpense	us-gaap:OtherNonoperatingIncomeExpense
Amounts recognized in other comprehensive income (loss)
Net periodic benefit cost	$ 5.6	$ 4.3	$ 11.3	$ 9.5
Other Postretirement Plans | Maximum
Net periodic benefit cost
Net periodic benefit cost	0.1	0.1	0.1	0.1
Amounts recognized in other comprehensive income (loss)
Net periodic benefit cost	$ 0.1	$ 0.1	$ 0.1	$ 0.1