Supplemental Guarantor Information (Details) - USD ($)	Jun. 30, 2015	Dec. 31, 2014
Credit facility
Financial statements
Maximum borrowing capacity	$ 300,000,000	$ 300,000,000
8.4 percent senior notes due May 2017
Financial statements
Interest rate stated percentage	8.40%
1.6 percent convertible senior notes due May 2018
Financial statements
Interest rate stated percentage	1.60%
0.25 percent convertible senior notes due June 2019
Financial statements
Interest rate stated percentage	0.25%
6.6 percent senior notes due May 2020
Financial statements
Interest rate stated percentage	6.60%
5.4 percent senior notes due October 2022
Financial statements
Interest rate stated percentage	5.40%