Summary of Significant Accounting Policies - Insurance Coverage (Details) (USD $)	12 Months Ended
	Dec. 31, 2012 levels	Dec. 31, 2011
Insurance Coverage [Line Items]
Levels of insurance coverage maintained by the Company	2
Amount in excess of primary insurance coverage	$ 100,000,000
Accrual for self-insurance liabilities	3,806,000	3,401,000
Other liability policies
Insurance Coverage [Line Items]
Primary limit of insurance coverage	1,000,000
Maritime employer's liability
Insurance Coverage [Line Items]
Primary limit of insurance coverage	10,000,000
Watercraft pollution policy
Insurance Coverage [Line Items]
Primary limit of insurance coverage	$ 5,000,000