Significant Accounting Policies (Narrative) (Details) (USD $)	0 Months Ended		12 Months Ended
	Sep. 07, 2012	Aug. 23, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Significant Accounting Policies [Line Items]
Stockholders' Equity Note, Stock Split, Conversion Ratio	1.5	1.5
Debt and Capital Lease Obligations			$ 240,407,000	$ 269,185,000
Depreciation expenses including capital leases			109,800,000	89,900,000	79,400,000
Increase in income from continuing operations					12,700,000
Increase in net income					7,700,000
Other intangible assets			8,100,000	8,100,000
Accumulated amortization of intangible assets			5,700,000	4,700,000
Amortization expense			900,000	900,000	900,000
Maximum self-insured long-term disability claims for employees per month				3,000
Last payment date of self-insured long-term disability claims for salaried employees					April 2, 2010
Last payment date of self-insured long-term disability claims for non-salaried employees				July 1, 2011
Self-insurance reserve			93,500,000	86,000,000
Long-term portions of self insurance reserve			59,500,000	50,100,000
Advertising expense			11,000,000	8,300,000	6,300,000
Long-term Debt, Fair Value			247,900,000	276,600,000
Trailers [Member]
Significant Accounting Policies [Line Items]
Estimated average useful lives of asset, years			15
Estimated previous useful lives of asset, years			12
Tractors [Member]
Significant Accounting Policies [Line Items]
Estimated average useful lives of asset, years			9
Estimated previous useful lives of asset, years			7
Group Health Claims [Member]
Significant Accounting Policies [Line Items]
Insurance maximum, per occurrence			400,000
Self insurance additional coverage			200,000
Workers Compensation Claims [Member]
Significant Accounting Policies [Line Items]
Insurance maximum, per occurrence			1,000,000
Cargo Loss And Damage Claims [Member]
Significant Accounting Policies [Line Items]
Insurance maximum, per occurrence			100,000
Bodily Injury And Property Damage [Member]
Significant Accounting Policies [Line Items]
Insurance maximum, per occurrence			$ 2,750,000