Condensed Consolidated Statements of Cash Flows (Parenthetical) (6.375% senior notes)
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Long-term debt and notes payable      
Interest rate of debt (as a percent) 6.375% 6.375% 6.375%
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Long-term debt and notes payable      
Interest rate of debt (as a percent) 6.375% 6.375% 6.375%