Fair Value Measurements and Derivative Instruments (Details) - Fair Value, Measurements, Nonrecurring - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Level 1
Assets:
Cash and cash equivalents	$ 121,565	$ 189,241
Total Assets	121,565	189,241
Liabilities:
Long-term debt (including current portion of long-term debt)	1,536,629	1,859,361
Total Liabilities	1,536,629	1,859,361
Level 2
Assets:
Cash and cash equivalents	0	0
Total Assets	0	0
Liabilities:
Long-term debt (including current portion of long-term debt)	7,358,380	6,902,053
Total Liabilities	7,358,380	6,902,053
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	121,565	189,241
Total Assets	121,565	189,241
Liabilities:
Long-term debt (including current portion of long-term debt)	8,618,285	8,391,301
Total Liabilities	8,618,285	8,391,301
Total Fair Value
Assets:
Cash and cash equivalents	121,565	189,241
Total Assets	121,565	189,241
Liabilities:
Long-term debt (including current portion of long-term debt)	8,895,009	8,761,414
Total Liabilities	$ 8,895,009	$ 8,761,414