Summary of Significant Accounting Policies - Long Lived Assets and Deferred Costs (Details) - USD ($)
|
12 Months Ended |
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
| Property Plant And Equipment [Line Items] |
|
|
|
| Annual growth factor management fees after 2018 |
0.30%
|
|
|
| Utilization rate of fleet |
99.20%
|
|
|
| Amortization of deferred drydock and special survey costs |
$ 14,727,000
|
$ 13,768,000
|
$ 13,340,000
|
| Amortization and write-off of deferred financing costs |
6,391,000
|
5,653,000
|
4,524,000
|
| Impairment loss |
50,565,000
|
0
|
0
|
| Vessels |
|
|
|
| Property Plant And Equipment [Line Items] |
|
|
|
| Scrap value of vessels per LWT |
340
|
|
|
| Capitalized interest costs |
$ 4,764,000
|
$ 5,843,000
|
$ 5,154,000
|
| Ocean-going vessels | Minimum |
|
|
|
| Property Plant And Equipment [Line Items] |
|
|
|
| Period of review of special costs of assets |
30 months
|
|
|
| Ocean-going vessels | Maximum |
|
|
|
| Property Plant And Equipment [Line Items] |
|
|
|
| Period of review of special costs of assets |
60 months
|
|
|
| Pushboats and barges | Maximum |
|
|
|
| Property Plant And Equipment [Line Items] |
|
|
|
| Period of review of special costs of assets |
96 months
|
|
|
| One vessel held and used |
|
|
|
| Property Plant And Equipment [Line Items] |
|
|
|
| Impairment loss |
$ 32,930,000
|
|
|