Equity (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Changes in total common stockholders' equity
Balance at the beginning of the period			$ 626,626
Common stock issued			1
Share-based compensation expense			1,732
Common stock dividends declared			(24,047)
Net income	$ 25,120	$ 33,650	36,540	$ 45,344
Balance at the end of the period	$ 640,852		$ 640,852