Fair Value of Financial Assets and Liabilities, Fair Value of Long-Term Debt (Details) - USD ($)
$ in Thousands
Sep. 30, 2016
Dec. 31, 2015
Carrying Amount    
Financial Liabilities, Balance Sheet Groupings [Abstract]    
Long-term debt, including current portion [1] $ 1,835,687 $ 1,538,522
Fair Value    
Financial Liabilities, Balance Sheet Groupings [Abstract]    
Long-term debt, including current portion [1] $ 2,065,982 $ 1,678,673
[1] Amounts reflect the classification of debt issuance costs as a deduction from the carrying amount of the related debt. See Note 2, Accounting Pronouncements for more information on the adoption of ASU 2015-03.