Debt (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Maturity of primary debt
2017	$ 890,000
Thereafter	3,500,000
Long-term debt total	4,448,888	4,693,004
6.15% senior notes due February 2018
Additional disclosure
Interest rate on senior note due (as a percent)	6.15%
Maturity of primary debt
Long-term debt total	968,708	967,490
9.25% senior notes due January 2019
Additional disclosure
Interest rate on senior note due (as a percent)	9.25%
Maturity of primary debt
Long-term debt total	1,125,000	1,125,000
5.00% senior notes due September 2020
Additional disclosure
Interest rate on senior note due (as a percent)	5.00%
Maturity of primary debt
Long-term debt total	697,648	697,343
4.625% senior notes due September 2021
Additional disclosure
Interest rate on senior note due (as a percent)	4.625%
Maturity of primary debt
Long-term debt total	$ 697,907	$ 697,667