Nature of Operations and Summary of Significant Accounting Policies (Details 2) (USD $)	Dec. 31, 2011 Y M	Dec. 31, 2010
Cash and Cash Equivalents
Cash and cash equivalents, maximum original maturity period (in months)	3
Restricted cash for workers compensation policy	$ 19,434,000	$ 21,775,000
Marketable Securities
Cost	501,181,000	675,480,000
Unrealized appreciation (depreciation)	164,000	(739,000)
Total marketable securities	501,345,000	674,741,000
Market Value	501,345,000	674,741,000
Maturities of marketable securities
Year 2012	202,454,000
Years 2013 through 2016	239,193,000
Years 2017 through 2021	1,509,000
Thereafter	58,189,000
Marketable securities	497,600,000
Maximum period for redemption (in year)	1
Investment as non-current	3,800,000
Commercial paper
Marketable Securities
Cost	4,555,000	5,002,000
Market Value	4,557,000	4,998,000
Bond and bond funds
Marketable Securities
Cost	496,170,000	669,786,000
Market Value	496,310,000	669,025,000
Asset backed securities
Marketable Securities
Cost	456,000	692,000
Market Value	$ 478,000	$ 718,000