Fair Value of Financial Assets and Liabilities, Fair Value of Long-Term Debt (Details) - USD ($) $ in Thousands |
Sep. 30, 2017 |
Dec. 31, 2016 |
|---|---|---|
| Carrying Amount | ||
| Financial Liabilities, Balance Sheet Groupings [Abstract] | ||
| Long-term debt, including current portion | $ 1,829,965 | $ 1,635,858 |
| Fair Value | ||
| Financial Liabilities, Balance Sheet Groupings [Abstract] | ||
| Long-term debt, including current portion | $ 1,954,618 | $ 1,741,502 |