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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.

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- Definition Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(02) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 7: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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