Summary of Significant Accounting Policies - Insurance Coverage (Details)	6 Months Ended
	Jun. 30, 2015 USD ($) levels	Dec. 31, 2014 USD ($)
Insurance Coverage [Line Items]
Levels of insurance coverage maintained by the Company | levels	3
Amount in excess of primary insurance coverage	$ 150,000,000
Accrual for self-insurance liabilities	7,848,000	$ 7,490,000
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