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, 2013	Dec. 31, 2012	Dec. 31, 2011
Accounting Policies [Abstract]
Capital lease incurred to acquire revenue equipment	$ 5,575	$ 0	$ 0
Revenue equipment acquired through partial non-monetary exchanges	27,711	34,759	33,463
Revenue equipment acquired through increase in current liabilities	32,336	14,034	0
Repurchased common stock issued under defined contribution plan	$ 0	$ 0	$ 17,307