Credit Facilities (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Unsecured term notes:
November 30, 2001, Series B Senior Notes, $250,000,000, 6.23% fixed, due November 30, 2011	$ 0	$ 250,000,000
Total debt	500,000,000	500,000,000
Less debt due within one year		250,000,000
Long-term debt, excluding current portion	500,000,000	250,000,000
Credit Facilities (Textual) [Abstract]
Weighted average interest rate on outstanding borrowings	4.01%	5.45%
Unsecured revolving line of credit facility	350,000,000
Line of credit facility interest rate	0.30%
Line of credit facility, interest rate at period end	0.60%
Line of Credit Facility Amount of Option to Increase Additional Borrowings	200,000,000
Outstanding Line of credit	0	0
Unused letter of credit outstanding due to workers' compensation and insurance reserve	53,703,000	50,419,000
4.67% Series C Senior Unsecured Notes [Member]
Unsecured term notes:
Long Term Debt	250,000,000	250,000,000
3.35% Series D and E Senior Unsecured Notes [Member]
Unsecured term notes:
Long Term Debt	250,000,000	0
Unsecured Senior Notes [Member] | 6.23% Series B Senior Notes [Member]
Credit Facilities, Long-Term Debt [Textual] [Abstract]
Face value of Senior Notes	250,000,000
Interest rate of Unsecured Senior Notes	6.23%
Due date of Senior Notes	Nov. 30, 2011
Unsecured Senior Notes [Member] | 4.67% Series C Senior Unsecured Notes [Member]
Credit Facilities, Long-Term Debt [Textual] [Abstract]
Face value of Senior Notes	250,000,000
Interest rate of Unsecured Senior Notes	4.67%
Due date of Senior Notes	Nov. 30, 2013
Unsecured Senior Notes [Member] | 3.35% Series D and E Senior Unsecured Notes [Member]
Credit Facilities, Long-Term Debt [Textual] [Abstract]
Face value of Senior Notes	$ 250,000,000
Interest rate of Unsecured Senior Notes	3.35%
Due date of Senior Notes	Nov. 30, 2016