Employee benefit plans (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Profit sharing, defined contribution and 401(k) plans [Abstract]
Percentage of eligible compensation contributed to defined contribution pension plan (in hundredths)		4.00%	4.00%
Maximum allowable employee contributions, employees under age 50	$ 17
Maximum allowable employee contributions as a percentage of eligible wages, employees under age 50 (in hundredths)	50.00%
Maximum allowable employee contributions, employees 50 years of age and older	22
Employer 401(k) plan matching contribution description	100% of the first 1% of wages contributed by employees and 50% of the next 5% of wages contributed
Profit sharing/cash bonus plans expense	16,361	18,500	22,751
Defined contribution pension plan expense	0	8,664	9,953
401(k) plan expense	6,226	5,636	6,312
Deferred compensation plan [Abstract]
Maximum percentage of base salary employees can defer (in hundredths)		100.00%
Maximum percentage of bonus employees can defer (in hundredths)		50.00%
Deferred compensation plan liability	2,391	2,920
Investments in company-owned life insurance policies that fund the deferred compensation plan	13,489	12,872
Voluntary employee beneficiary association (VEBA) trust [Abstract]
Contributions to VEBA trust	36,792	39,400	40,300
Amount by which prepaid balance in VEBA trust exceeds the liability for IBNR medical claims	$ 361	$ 959