Supplemental Guarantor Information (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
5.4 percent senior notes due January 2015
Financial statements
Interest rate stated percentage	5.40%	5.40%
8.4 percent senior notes due May 2017
Financial statements
Interest rate stated percentage	8.40%	8.40%
1.6 percent convertible senior notes due May 2018
Financial statements
Interest rate stated percentage	1.60%	1.60%	1.60%
0.25 percent convertible senior notes due June 2019
Financial statements
Interest rate stated percentage	0.25%	0.25%	0.25%
6.6 percent senior notes due May 2020
Financial statements
Interest rate stated percentage	6.60%	6.60%
5.4 percent senior notes due October 2022
Financial statements
Interest rate stated percentage	5.40%	5.40%
Credit facility
Financial statements
Maximum borrowing capacity	300.0