Summary of Significant Accounting Policies - Long Lived Assets and Deferred Costs (Details) - USD ($) |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2016 |
Dec. 31, 2015 |
Dec. 31, 2014 |
|
| Property Plant And Equipment [Line Items] | |||
| Annual growth factor management fees after 2017 | 0.80% | ||
| Utilization rate of fleet | 99.20% | ||
| Amortization of deferred drydock and special survey costs | $ 13,768,000 | $ 13,340,000 | $ 12,263,000 |
| Amortization and write-off of deferred financing costs | 5,653,000 | 4,524,000 | 4,061,000 |
| Vessels | |||
| Property Plant And Equipment [Line Items] | |||
| Scrap value of vessels per LWT | 340 | ||
| Capitalized interest costs | $ 5,843,000 | $ 5,154,000 | $ 2,334,000 |
| 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 | ||
| Push boats and barges | |||
| Property Plant And Equipment [Line Items] | |||
| Period of review of special costs of assets | 84 months | ||