Consolidated Balance Sheets (unaudited) (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Property, plant and equipment, accumulated depreciation	$ 2,318,227	$ 2,250,950
Preferred stock, authorized shares (in shares)	10,000,000	10,000,000
Preferred stock, issued shares (in shares)	0	0
Common stock, authorized shares (in shares)	200,000,000	200,000,000
Common stock, issued shares (in shares)	107,458,641	102,565,266
Common stock, outstanding shares (in shares)	107,458,641	102,565,266