Pension Benefits - Fair Value of Plan Assets (Details) - Pension Plan - USD ($) $ in Thousands	Dec. 27, 2020	Dec. 29, 2019	Dec. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	$ 1,585,221	$ 1,648,667	$ 1,410,151
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	1,585,221
Recurring | U.S. Equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	28,002	55,011
Recurring | International Equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	34,025	38,231
Recurring | Mutual Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	29,011	46,276
Recurring | Registered Investment Companies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	66,923	52,582
Recurring | Common Collective Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	643,443	575,738
Recurring | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	646,330	574,756
Recurring | U.S. Treasury and Other Government Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	42,111	196,009
Recurring | Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	40,150	42,812
Recurring | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	10,693	11,745
Recurring | Cash and Cash Equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	5,481	19,097
Recurring | Private Equity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	9,770	11,345
Recurring | Hedge Fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	29,282	25,065
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	157,961	192,100
Recurring | Level 1 | U.S. Equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	28,002	55,011
Recurring | Level 1 | International Equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	34,025	38,231
Recurring | Level 1 | Mutual Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	29,011	46,276
Recurring | Level 1 | Registered Investment Companies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	66,923	52,582
Recurring | Level 1 | Common Collective Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 1 | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 1 | U.S. Treasury and Other Government Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 1 | Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 1 | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 1 | Cash and Cash Equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 1 | Private Equity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 1 | Hedge Fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	739,284	825,322
Recurring | Level 2 | U.S. Equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 2 | International Equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 2 | Mutual Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 2 | Registered Investment Companies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 2 | Common Collective Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 2 | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	646,330	574,756
Recurring | Level 2 | U.S. Treasury and Other Government Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	42,111	196,009
Recurring | Level 2 | Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	40,150	42,812
Recurring | Level 2 | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	10,693	11,745
Recurring | Level 2 | Cash and Cash Equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 2 | Private Equity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 2 | Hedge Fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 3 | U.S. Equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 3 | International Equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 3 | Mutual Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 3 | Registered Investment Companies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 3 | Common Collective Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 3 | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 3 | U.S. Treasury and Other Government Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 3 | Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 3 | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 3 | Cash and Cash Equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 3 | Private Equity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Level 3 | Hedge Fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Investment Measured at Net Asset Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	687,976	631,245
Recurring | Investment Measured at Net Asset Value | U.S. Equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Investment Measured at Net Asset Value | International Equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Investment Measured at Net Asset Value | Mutual Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Investment Measured at Net Asset Value | Registered Investment Companies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Investment Measured at Net Asset Value | Common Collective Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	643,443	575,738
Recurring | Investment Measured at Net Asset Value | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Investment Measured at Net Asset Value | U.S. Treasury and Other Government Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Investment Measured at Net Asset Value | Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Investment Measured at Net Asset Value | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	0	0
Recurring | Investment Measured at Net Asset Value | Cash and Cash Equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	5,481	19,097
Recurring | Investment Measured at Net Asset Value | Private Equity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	9,770	11,345
Recurring | Investment Measured at Net Asset Value | Hedge Fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined benefits plan, plan assets	$ 29,282	$ 25,065