Equity (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Changes in total common stockholders' equity
Balance at December 31, 2017	$ 693,462
Common stock issued	1
Share-based compensation expense	635
Repurchase of common stock	(1,239)	$ (1,119)
Common stock dividends declared	(9,003)
Net (loss) income	(2,545)	$ 1,132
Balance at March 31, 2018	$ 681,311