Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Consolidated Balance Sheets
Property, plant and equipment, accumulated depreciation (in dollars)	$ 2,161,681	$ 2,125,286
Preferred stock, authorized shares	10,000,000	10,000,000
Preferred stock, issued shares	0	0
Common stock, authorized shares	200,000,000	200,000,000
Common stock, issued shares	99,044,053	97,928,403
Common stock, outstanding shares	99,044,053	97,928,403