Employee Benefit Plans and Other Postretirement Benefits - Fair Value Measurements of Plan Assets (Parenthetical) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
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