Equity - Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Equity activity
Balance at beginning of period			$ 659,202
Net income	$ 22,411	$ 19,647	64,449	$ 55,127
Proceeds from common stock offering, net of offering costs			78,137
Stock-based compensation expense			3,149
Performance-based stock units			(41)
Stock option exercise			159
Reclassification adjustment	(6)	$ (9)	(39)	$ (26)
Common stock dividends			(62,211)
Other			(2,174)
Balance at end of period	$ 740,631		$ 740,631