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, 2018	Dec. 31, 2017		Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		$ 873.2	$ 954.8		$ 830.4
Cash and Short-term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		12.0	608.1	[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		154.5	84.1
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		238.8	5.6
Government Bonds (Corporate and Government Bonds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		164.4
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		64.4
Fixed Income (Corporate and Government Bonds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		234.5	41.3
Private Equity Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets	[2]	3.3	4.2
Total Securities at Fair Value
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		871.9	954.6
Receivables and Accrued Expenses
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		1.3	0.2
International equities (Mutual Funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets			56.1
Fixed Income (Mutual Funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets			95.4
Fixed Income (Common/Collective Trust Funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets			59.8
Quoted prices in active markets for identical assets (Level 1) | Cash and Short-term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		0.4	602.4	[1]
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) | 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		136.9	0.0
Quoted prices in active markets for identical assets (Level 1) | Government Bonds (Corporate and Government Bonds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		155.0
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
Quoted prices in active markets for identical assets (Level 1) | Fixed Income (Corporate and Government Bonds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		95.7	41.3
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	[2]	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		388.0	795.2
Quoted prices in active markets for identical assets (Level 1) | International equities (Mutual Funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets			56.1
Quoted prices in active markets for identical assets (Level 1) | Fixed Income (Mutual Funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets			95.4
Quoted prices in active markets for identical assets (Level 1) | Fixed Income (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
Significant other observable inputs (Level 2) | Cash and Short-term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		11.6	5.7	[1]
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		154.5	84.1
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		17.8	5.6
Significant other observable inputs (Level 2) | Government Bonds (Corporate and Government Bonds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		9.4
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		64.4
Significant other observable inputs (Level 2) | Fixed Income (Corporate and Government Bonds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		138.8	0.0
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	[2]	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		396.5	155.2
Significant other observable inputs (Level 2) | International equities (Mutual Funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets			0.0
Significant other observable inputs (Level 2) | Fixed Income (Mutual Funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets			0.0
Significant other observable inputs (Level 2) | Fixed Income (Common/Collective Trust Funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets			59.8
Significant unobservable input (Level 3) | Cash and Short-term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets		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) | 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) | Government 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
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
Significant unobservable input (Level 3) | Fixed Income (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	[2]	3.3	4.2
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		3.3	4.2
Significant unobservable input (Level 3) | International equities (Mutual Funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets			0.0
Significant unobservable input (Level 3) | Fixed Income (Mutual Funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets			0.0
Significant unobservable input (Level 3) | Fixed Income (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
Net Asset Value (NAV) | Cash and Short-term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets	[3]	0.0
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	[3]	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	[3]	84.1	0.0
Net Asset Value (NAV) | Government Bonds (Corporate and Government Bonds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets	[3]	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	[3]	0.0
Net Asset Value (NAV) | Fixed Income (Corporate and Government Bonds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets	[3]	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	[2],[3]	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	[3]	$ 84.1	0.0
Net Asset Value (NAV) | International equities (Mutual Funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets	[3]		0.0
Net Asset Value (NAV) | Fixed Income (Mutual Funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets	[3]		0.0
Net Asset Value (NAV) | Fixed Income (Common/Collective Trust Funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of plan assets	[3]		$ 0.0

[1] As of December 31, 2017, the cash and short-term investments asset category contained both short-term investments and the Boise pension plan assets classified as cash. Of the $551.3 million of Boise pension plan assets classified as Level 1 at December 31, 2017, $449.8 million of the pension plan assets were reclassified to Level 2 equities and fixed income securities on or about January 5, 2018 after consolidating with the PCA pension plan assets under one trustee.

[2] 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.

[3] 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.