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Employee Benefit Plans and Other Postretirement Benefits - Summary of Changes in Pension Plans' Level 3 Assets (Details) - Pension Plans $ in Millions | 12 Months Ended
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Dec. 31, 2018 USD ($)
Level 3 fair value of pension plans' assets [Roll Forward] |  |
Plan assets at fair value at beginning of period | $ 954.8 |
Fair value of plan assets at plan year end | 873.2 |
Private Equity Securities |  |
Level 3 fair value of pension plans' assets [Roll Forward] |  |
Plan assets at fair value at beginning of period | 4.2 | [1]
Fair value of plan assets at plan year end | 3.3 | [1]
Significant unobservable input (Level 3) | Private Equity Securities |  |
Level 3 fair value of pension plans' assets [Roll Forward] |  |
Plan assets at fair value at beginning of period | 4.2 | [1]
Sales | (0.9) |
Fair value of plan assets at plan year end | $ 3.3 | [1]
[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 $5.0 million of the commitment unfunded at December 31, 2018.

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