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Employee Benefit Plans and Other Postretirement Benefits - Fair Value Measurements of Plan Assets (Parenthetical) (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
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Quoted prices in active markets for identical assets (Level 1) | Boise Pension Plan | Cash and Short-term Investments |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cash and short-term investments |  | $ 551.3
Significant other observable inputs (Level 2) | Boise Pension Plan | Fixed Income Securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cash and short-term investments |  | $ 449.8
Significant unobservable input (Level 3) | Private Equity Securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investment commitment | $ 15.0 |
Unfunded portion of investment commitment | $ 5.0 |

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- Definition Amount of investment within defined benefit plan asset category. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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