Financial Instruments - Derivatives and Hedging (Fair Value) (Details) - Interest Rate Swap - USD ($) $ in Thousands |
Mar. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Derivatives, Fair Value [Line Items] | ||
| Gross derivative assets | $ 29,193 | $ 24,420 |
| Gross derivative liabilities | 0 | 0 |
| Net derivative assets | $ 29,193 | $ 24,420 |