Accrued Workers' Compensation (Worker's Compensation Expense) (Details) (Workers' Compensation [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Workers' Compensation [Member]
Accrued Workers' Compensation [Line Items]
Service cost	$ 1,993	$ 2,059	$ 727
Interest cost	2,400	1,799	675
Net amortization	(453)	(493)	(1,860)
Recognized Net Gain (Loss) Due to Curtailments	3,022	0	0
Total net benefit	6,962	3,365	(458)
Traumatic injury claims and assessments	26,565	16,979	9,263
Total workers' compensation expense	$ 33,527	$ 20,344	$ 8,805