Condensed Consolidated Balance Sheets (unaudited) (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Property, plant and equipment, accumulated depreciation	$ 2,475,562	$ 2,444,348
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)	108,745,265	108,583,413
Common stock, outstanding shares (in shares)	108,745,265	108,583,413