Commitments and Contingencies (Details) - USD ($)	Jun. 30, 2017	Dec. 31, 2016
Loss Contingencies [Line Items]
Reserves for estimated uninsured losses	$ 2,900,000	$ 8,900,000
Insurance Claims
Loss Contingencies [Line Items]
Liability and cargo insurance coverage for claims	1,000,000
Cargo Claims
Loss Contingencies [Line Items]
Liability and cargo insurance coverage for claims	100,000
Uninsured Risk
Loss Contingencies [Line Items]
Reserves for estimated uninsured losses	26,100,000	21,500,000
Legal Reserve [Member]
Loss Contingencies [Line Items]
Reserves for estimated uninsured losses	$ 13,900,000	$ 10,400,000