Noncontrolling interests (Details) (USD $)	3 Months Ended		6 Months Ended				6 Months Ended
	Jul. 01, 2012	Jul. 03, 2011	Jul. 01, 2012	Jul. 03, 2011	Jul. 01, 2012 Common Stock [Member]	Dec. 31, 2011 Common Stock [Member]	Jul. 01, 2012 Additional Paid-in Capital [Member]	Jul. 01, 2012 Retained Earnings [Member]	Jul. 01, 2012 Accumulated Other Comprehensive Income (Loss) [Member]	Jul. 01, 2012 Treasury Stock [Member]	Jul. 01, 2012 Noncontrolling Interest [Member]
Balance			$ 768,020,000		$ 2,900,000	$ 2,900,000	$ 335,100,000	$ 963,100,000	$ (45,300,000)	$ (514,200,000)	$ 26,400,000
Net income	20,239,000	17,156,000	38,847,000	33,846,000				37,700,000			1,100,000
Other comprehensive income			(2,600,000)						(2,300,000)		(300,000)
Dividends declared			(1,800,000)					(1,800,000)
Dividends paid to non-controlling interest			(3,249,000)	0							(3,200,000)
Capital contributions by non-controlling interest			800,000								800,000
Employee benefit transactions			2,400,000				2,400,000
Income tax benefit arising from employee stock option			200,000				200,000
Stock based compensation			1,000,000				1,000,000
Purchase of common stock			(6,000,000)							(6,000,000)
Balance	$ 797,571,000		$ 797,571,000		$ 2,900,000	$ 2,900,000	$ 338,700,000	$ 999,100,000	$ (47,700,000)	$ (520,200,000)	$ 24,800,000