Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Stockholders' equity:
Common stock, shares issued	198,316,808	196,564,907	195,843,297
Treasury shares	538,921	538,921	538,921
Common stock, shares authorized	500,000,000	500,000,000	500,000,000
Common stock, par value	$ 1.00	$ 1.00	$ 1.00