Financial Instruments - Derivatives and Hedging (Cash Flow Hedging Relationship) (Details) - USD ($) $ in Thousands |
3 Months Ended | |
|---|---|---|
Mar. 31, 2018 |
Mar. 31, 2017 |
|
| Derivative Instruments and Hedging Activities Disclosure [Abstract] | ||
| Change in unrealized gain on interest rate swaps | $ 7,234 | $ 2,157 |
| Amortization (accretion) of interest rate swaps to interest expense | (2,461) | 462 |
| Change in unrealized gain on interest rate swaps, net | $ 4,773 | $ 2,619 |