Consolidated Balance Sheets (unaudited) (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Statement of Financial Position [Abstract]
Property, plant and equipment, accumulated depreciation	$ 2,387,013	$ 2,339,319
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)	108,187,063	107,460,406
Common stock, outstanding shares (in shares)	108,187,063	107,460,406