Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2018	Dec. 31, 2017
Consolidated Balance Sheets
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, authorized (in shares)	300,000,000	300,000,000
Common stock, issued (in shares)	139,620,801	139,599,454
Common stock outstanding (in shares)	136,642,060	139,588,782
Treasury stock (in shares)	2,978,741	10,672