Fair Value Measurements and Derivative Instruments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
Assets:
Cash and cash equivalents	$ 205,174	$ 194,855	$ 212,209	$ 262,186
Fair Value, Measurements, Nonrecurring | Total
Assets:
Cash and cash equivalents	205,174	194,855
Total Assets	205,174	194,855
Liabilities:
Long-term debt (including current portion of long-term debt)	8,571,141	8,859,310
Total Liabilities	8,571,141	8,859,310
Fair Value, Measurements, Nonrecurring | Level 1
Assets:
Cash and cash equivalents	205,174	194,855
Total Assets	205,174	194,855
Liabilities:
Long-term debt (including current portion of long-term debt)	3,215,109	3,917,398
Total Liabilities	3,215,109	3,917,398
Fair Value, Measurements, Nonrecurring | Level 2
Liabilities:
Long-term debt (including current portion of long-term debt)	5,356,032	4,941,912
Total Liabilities	$ 5,356,032	$ 4,941,912