Employee Benefit Plans and Other Postretirement Benefits - Schedule of Fair Value Measurements of Plan Assets by Major Asset Category (Details) - Pension Plans - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		$ 1,300.7	$ 1,073.8	$ 873.2
Accrued Expenses and Receivables
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		1.6	(2.4)
Defined Benefit Plan, Cash
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		25.1	14.9
International equities (Common/Collective Trust Funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		368.1	286.1
Domestic equities (Common/Collective Trust Funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		276.9	240.5
Corporate Bonds (Corporate and Government Bonds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		318.3	70.6
U.S. Government Agency Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		243.4	160.7
Fixed Income Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		49.4	295.0
Municipal Bonds (Corporate and Government Bonds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		15.9	5.8
Private Equity Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets	[1]	2.0	2.6
Total Securities at Fair Value
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		1,299.1	1,076.2
Quoted prices in active markets for identical assets (Level 1) | Defined Benefit Plan, Cash
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		0.0	0.0
Quoted prices in active markets for identical assets (Level 1) | International equities (Common/Collective Trust Funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		198.4	163.0
Quoted prices in active markets for identical assets (Level 1) | Domestic equities (Common/Collective Trust Funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		0.0	0.0
Quoted prices in active markets for identical assets (Level 1) | Corporate Bonds (Corporate and Government Bonds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		0.0	0.0
Quoted prices in active markets for identical assets (Level 1) | U.S. Government Agency Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		0.0	0.0
Quoted prices in active markets for identical assets (Level 1) | Fixed Income Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		0.0	140.4
Quoted prices in active markets for identical assets (Level 1) | Municipal Bonds (Corporate and Government Bonds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		0.0	0.0
Quoted prices in active markets for identical assets (Level 1) | Private Equity Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets	[1]	0.0	0.0
Quoted prices in active markets for identical assets (Level 1) | Total Securities at Fair Value
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		198.4	303.4
Significant other observable inputs (Level 2) | Defined Benefit Plan, Cash
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		25.1	14.9
Significant other observable inputs (Level 2) | International equities (Common/Collective Trust Funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		32.8	20.8
Significant other observable inputs (Level 2) | Domestic equities (Common/Collective Trust Funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		276.9	240.5
Significant other observable inputs (Level 2) | Corporate Bonds (Corporate and Government Bonds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		318.3	70.6
Significant other observable inputs (Level 2) | U.S. Government Agency Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		243.4	160.7
Significant other observable inputs (Level 2) | Fixed Income Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		49.4	154.6
Significant other observable inputs (Level 2) | Municipal Bonds (Corporate and Government Bonds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		15.9	5.8
Significant other observable inputs (Level 2) | Private Equity Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets	[1]	0.0	0.0
Significant other observable inputs (Level 2) | Total Securities at Fair Value
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		961.8	667.9
Significant unobservable input (Level 3) | Defined Benefit Plan, Cash
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		0.0	0.0
Significant unobservable input (Level 3) | International equities (Common/Collective Trust Funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		0.0	0.0
Significant unobservable input (Level 3) | Domestic equities (Common/Collective Trust Funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		0.0	0.0
Significant unobservable input (Level 3) | Corporate Bonds (Corporate and Government Bonds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		0.0	0.0
Significant unobservable input (Level 3) | U.S. Government Agency Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		0.0	0.0
Significant unobservable input (Level 3) | Fixed Income Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		0.0	0.0
Significant unobservable input (Level 3) | Municipal Bonds (Corporate and Government Bonds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		0.0	0.0
Significant unobservable input (Level 3) | Private Equity Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets	[1]	2.0	2.6
Total fair value of plan assets		2.0	2.6
Significant unobservable input (Level 3) | Total Securities at Fair Value
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		2.0	2.6
Net Asset Value (NAV) | Defined Benefit Plan, Cash
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets	[2]	0.0	0.0
Net Asset Value (NAV) | International equities (Common/Collective Trust Funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets	[2]	136.9	102.3
Net Asset Value (NAV) | Domestic equities (Common/Collective Trust Funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets	[2]	0.0	0.0
Net Asset Value (NAV) | Corporate Bonds (Corporate and Government Bonds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets	[2]	0.0	0.0
Net Asset Value (NAV) | U.S. Government Agency Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets	[2]	0.0	0.0
Net Asset Value (NAV) | Fixed Income Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets	[2]	0.0	0.0
Net Asset Value (NAV) | Municipal Bonds (Corporate and Government Bonds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets	[2]	0.0	0.0
Net Asset Value (NAV) | Private Equity Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets	[1],[2]	0.0	0.0
Net Asset Value (NAV) | Total Securities at Fair Value
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets	[2]	$ 136.9	$ 102.3

[1] Investments in this category are invested in the Pantheon Global Secondary Fund IV, LP. The fund specializes in investments in the private equity secondary market and occasionally directly in private companies to maximize capital growth. Fund investments are carried at fair value as determined quarterly using the market approach to estimate the fair value of private investments. The market approach utilizes prices and other relevant information generated by market transactions, type of security, size of the position, degree of liquidity, restrictions on the disposition, latest round of financing data, current financial position, and operating results, among other factors. In circumstances where fair values are not provided with respect to any of the company's fund investments, the investment advisor will seek to determine the fair value of such investments based on information provided by the general partners or managers of such funds or from other sources. Audited financial statements are provided by fund management annually. Notwithstanding the above, the variety of valuation bases adopted and quality of management data of the ultimate underlying investee companies means that there are inherent difficulties in determining the value of the investments. Amounts realized on the sale of these investments may differ from the calculated values. Boise had originally committed to a $15.0 million investment, with $2.0 million of the commitment unfunded at December 31, 2020.

[2] In accordance with ASC 820, Fair Value Measurement, certain investments that do not have readily-determinable fair values are measured at fair value using the net asset value (NAV) per share practical expedient and are not classified within the fair value hierarchy.