Fair Value Measurements and Derivative Instruments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Assets:
Cash and cash equivalents	$ 215,888	$ 194,855	$ 277,533	$ 262,186
Fair Value, Measurements, Nonrecurring | Total
Assets:
Cash and cash equivalents	215,888	194,855
Total Assets	215,888	194,855
Liabilities:
Long-term debt (including current portion of long-term debt)	8,795,336	8,859,310
Total Liabilities	8,795,336	8,859,310
Fair Value, Measurements, Nonrecurring | Level 1
Assets:
Cash and cash equivalents	215,888	194,855
Total Assets	215,888	194,855
Liabilities:
Long-term debt (including current portion of long-term debt)	3,845,459	3,917,398
Total Liabilities	3,845,459	3,917,398
Fair Value, Measurements, Nonrecurring | Level 2
Liabilities:
Long-term debt (including current portion of long-term debt)	4,949,877	4,941,912
Total Liabilities	$ 4,949,877	$ 4,941,912