Fair Value Measurements and Derivative Instruments (Details) - Fair Value, Measurements, Nonrecurring - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Level 1
Assets:
Cash and cash equivalents	$ 120,112	$ 132,603
Total Assets	120,112	132,603
Liabilities:
Long-term debt (including current portion of long-term debt)	0	0
Total Liabilities	0	0
Level 2
Assets:
Cash and cash equivalents	0	0
Total Assets	0	0
Liabilities:
Long-term debt (including current portion of long-term debt)	8,038,092	9,859,266
Total Liabilities	8,038,092	9,859,266
Level 3
Assets:
Cash and cash equivalents	0	0
Total Assets	0	0
Liabilities:
Long-term debt (including current portion of long-term debt)	0	0
Total Liabilities	0	0
Total Carrying Amount
Assets:
Cash and cash equivalents	120,112	132,603
Total Assets	120,112	132,603
Liabilities:
Long-term debt (including current portion of long-term debt)	7,506,312	9,347,051
Total Liabilities	7,506,312	9,347,051
Total Fair Value
Assets:
Cash and cash equivalents	120,112	132,603
Total Assets	120,112	132,603
Liabilities:
Long-term debt (including current portion of long-term debt)	8,038,092	9,859,266
Total Liabilities	$ 8,038,092	$ 9,859,266