Employee Benefit Plans and Other Postretirement Plans - Fair Value Measurements of Plan Assets (Details) - USD ($)		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Company contributions		$ 57,100,000	$ 57,100,000	$ 400,000
Fair value of plan assets		764,400,000	764,400,000		$ 830,400,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period		764,400,000
Fair value of plan assets at plan year end		830,400,000	764,400,000
Significant unobservable input (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets		6,400,000	6,400,000		5,700,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period		6,400,000
Sales		(700,000)
Fair value of plan assets at plan year end		5,700,000	6,400,000
Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[1]	4,100,000	4,100,000		1,500,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[1]	4,100,000
Fair value of plan assets at plan year end	[1]	1,500,000	4,100,000
Short-term Investments | Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[1]	0	0		0
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[1]	0
Fair value of plan assets at plan year end	[1]	0	0
Short-term Investments | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[1]	4,100,000	4,100,000		1,500,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[1]	4,100,000
Fair value of plan assets at plan year end	[1]	1,500,000	4,100,000
Short-term Investments | Significant unobservable input (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[1]	0	0		0
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[1]	0
Fair value of plan assets at plan year end	[1]	0	0
Domestic equities (Mutual Funds)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[2]	47,100,000	47,100,000		65,700,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[2]	47,100,000
Fair value of plan assets at plan year end	[2]	65,700,000	47,100,000
Domestic equities (Mutual Funds) | Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[2]	47,100,000	47,100,000		65,700,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[2]	47,100,000
Fair value of plan assets at plan year end	[2]	65,700,000	47,100,000
Domestic equities (Mutual Funds) | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[2]	0	0		0
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[2]	0
Fair value of plan assets at plan year end	[2]	0	0
Domestic equities (Mutual Funds) | Significant unobservable input (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[2]	0	0		0
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[2]	0
Fair value of plan assets at plan year end	[2]	0	0
International equities (Mutual Funds)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[2]	50,700,000	50,700,000		68,700,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[2]	50,700,000
Fair value of plan assets at plan year end	[2]	68,700,000	50,700,000
International equities (Mutual Funds) | Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[2]	50,700,000	50,700,000		68,700,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[2]	50,700,000
Fair value of plan assets at plan year end	[2]	68,700,000	50,700,000
International equities (Mutual Funds) | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[2]	0	0		0
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[2]	0
Fair value of plan assets at plan year end	[2]	0	0
International equities (Mutual Funds) | Significant unobservable input (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[2]	0	0		0
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[2]	0
Fair value of plan assets at plan year end	[2]	0	0
Real Estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[2]	8,500,000	8,500,000		9,600,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[2]	8,500,000
Fair value of plan assets at plan year end	[2]	9,600,000	8,500,000
Real Estate | Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[2]	8,500,000	8,500,000		9,600,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[2]	8,500,000
Fair value of plan assets at plan year end	[2]	9,600,000	8,500,000
Real Estate | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[2]	0	0		0
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[2]	0
Fair value of plan assets at plan year end	[2]	0	0
Real Estate | Significant unobservable input (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[2]	0	0		0
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[2]	0
Fair value of plan assets at plan year end	[2]	0	0
Fixed Income (Mutual Funds)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[2]	156,500,000	156,500,000		173,200,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[2]	156,500,000
Fair value of plan assets at plan year end	[2]	173,200,000	156,500,000
Fixed Income (Mutual Funds) | Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[2]	156,500,000	156,500,000		108,400,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[2]	156,500,000
Fair value of plan assets at plan year end	[2]	108,400,000	156,500,000
Fixed Income (Mutual Funds) | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[2]	0	0		64,800,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[2]	0
Fair value of plan assets at plan year end	[2]	64,800,000	0
Fixed Income (Mutual Funds) | Significant unobservable input (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[2]	0	0		0
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[2]	0
Fair value of plan assets at plan year end	[2]	0	0
Domestic equities (Common/collective trusts)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[1]	104,400,000	104,400,000		107,900,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[1]	104,400,000
Fair value of plan assets at plan year end	[1]	107,900,000	104,400,000
Domestic equities (Common/collective trusts) | Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[1]	0	0		0
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[1]	0
Fair value of plan assets at plan year end	[1]	0	0
Domestic equities (Common/collective trusts) | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[1]	104,400,000	104,400,000		107,900,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[1]	104,400,000
Fair value of plan assets at plan year end	[1]	107,900,000	104,400,000
Domestic equities (Common/collective trusts) | Significant unobservable input (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[1]	0	0		0
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[1]	0
Fair value of plan assets at plan year end	[1]	0	0
International equities (Common/Collective Trust Funds)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[1]	121,600,000	121,600,000		124,500,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[1]	121,600,000
Fair value of plan assets at plan year end	[1]	124,500,000	121,600,000
International equities (Common/Collective Trust Funds) | Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[1]	0	0		0
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[1]	0
Fair value of plan assets at plan year end	[1]	0	0
International equities (Common/Collective Trust Funds) | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[1]	121,600,000	121,600,000		124,500,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[1]	121,600,000
Fair value of plan assets at plan year end	[1]	124,500,000	121,600,000
International equities (Common/Collective Trust Funds) | Significant unobservable input (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[1]	0	0		0
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[1]	0
Fair value of plan assets at plan year end	[1]	0	0
Fixed Income (Common/Collective Trust Funds) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[1]	263,000,000	263,000,000		271,600,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[1]	263,000,000
Fair value of plan assets at plan year end	[1]	271,600,000	263,000,000
Fixed Income (Common/Collective Trust Funds) [Member] | Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[1]	0	0		0
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[1]	0
Fair value of plan assets at plan year end	[1]	0	0
Fixed Income (Common/Collective Trust Funds) [Member] | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[1]	263,000,000	263,000,000		271,600,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[1]	263,000,000
Fair value of plan assets at plan year end	[1]	271,600,000	263,000,000
Fixed Income (Common/Collective Trust Funds) [Member] | Significant unobservable input (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[1]	0	0		0
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[1]	0
Fair value of plan assets at plan year end	[1]	0	0
Private Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[3]	6,400,000	6,400,000		5,700,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[3]	6,400,000
Fair value of plan assets at plan year end	[3]	5,700,000	6,400,000
Private Equity Securities | Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[3]	0	0		0
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[3]	0
Fair value of plan assets at plan year end	[3]	0	0
Private Equity Securities | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[3]	0	0		0
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[3]	0
Fair value of plan assets at plan year end	[3]	0	0
Private Equity Securities | Significant unobservable input (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets	[3]	6,400,000	6,400,000		5,700,000
Investment commitment					15,000,000
Unfunded portion of investment commitment					5,000,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	[3]	6,400,000
Fair value of plan assets at plan year end	[3]	5,700,000	6,400,000
Total Securities at Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets		762,300,000	762,300,000		828,400,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period		762,300,000
Fair value of plan assets at plan year end		828,400,000	762,300,000
Total Securities at Fair Value | Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets		262,800,000	262,800,000		252,400,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period		262,800,000
Fair value of plan assets at plan year end		252,400,000	262,800,000
Total Securities at Fair Value | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets		493,100,000	493,100,000		570,300,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period		493,100,000
Fair value of plan assets at plan year end		570,300,000	493,100,000
Total Securities at Fair Value | Significant unobservable input (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets		6,400,000	6,400,000		5,700,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period		6,400,000
Fair value of plan assets at plan year end		5,700,000	6,400,000
Receivables and Accruals, Net [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of plan assets		2,100,000	2,100,000		$ 2,000,000
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period		2,100,000
Fair value of plan assets at plan year end		$ 2,000,000	$ 2,100,000

[1] Investments in common/collective trust funds valued using net asset values (NAV) provided by the administrator of the funds. The NAV is based on the value of the underlying assets owned by the fund, minus its liabilities, and then divided by the number of units outstanding. While the underlying assets are actively traded on an exchange, the funds are not. There are currently no redemption restrictions on these investments. There are certain funds with one-day redeemable notice.

[2] Investments in mutual funds valued at quoted market values on the last business day of the fiscal year.

[3] 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, 2016.