Principal Accounting Policies Principal Accounting Policies (Schedule of Non-cash Investing and Financing Activities) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Accounting Policies [Abstract]
Capital lease incurred to acquire revenue equipment			$ 55,534
Revenue equipment acquired through partial non-monetary exchanges	34,759	33,463	0
Revenue Equipment Acquired Through Increase in Accrued Liabilities	14,034	0	0
Repurchased common stock issued under defined contribution plan	$ 0	$ 17,307	$ 36,763