Workers' Compensation Expense (Worker's Compensation Expense) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Curtailments			$ 424
Workers' Compensation [Member]
Service cost	71	246	1,039	439
Interest cost	399	304	1,079	558
Net amortization	(851)	(160)	(574)	(261)
Curtailments	1,933		1,933
Net benefit cost (credit)	1,552	390	3,477	736
Traumatic injury claims and assessments	6,423	3,324	11,599	5,649
Total workers' compensation expense	$ 7,975	$ 3,714	$ 15,076	$ 6,385