Equity (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Changes in total stockholders' equity
Balance at December 31, 2013			$ 598,756
Common stock issued			1
Share-based compensation expense			1,476
Common stock dividends paid			(23,295)
Net income	33,650	29,151	45,344	41,588
Balance at September 30, 2014	$ 622,282		$ 622,282