Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 25, 2016
Current assets
Accounts receivable, allowance for doubtful accounts	$ 31,400	$ 16,800
Less accumulated depreciation	$ 422,052	$ 383,713
Shareholders' equity
Preference stock, par value (in dollars per share)	$ 2.5	$ 2.5
Preference stock, shares authorized (in shares)	5,000,000	5,000,000
Preference stock, shares issued (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.5	$ 0.5
Common stock, shares authorized (in shares)	600,000,000	600,000,000
Common stock, shares issued (in shares)	209,694,630	209,694,630
Treasury stock, at cost, shares (in shares)	85,244,923	85,207,677