Equity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance (in shares)		48,065			48,065
Balance	$ 714,129	$ 730,157	$ 688,853	$ 699,221	$ 730,157	$ 699,221
Net income	16,996	(7,640)	14,805	(762)	$ 9,356	14,043
Issuance of common stock	1,675	3,179	737	636
Repurchase of common stock	(129)	(2,074)	(124)	(1,239)
Dividends paid on common stock	$ (9,507)	$ (9,493)	$ (9,014)	$ (9,003)
Dividends paid on common stock (in dollars per share)	$ 0.1975	$ 0.1975	$ 0.1875	$ 0.1875
Balance (in shares)	48,140				48,140
Balance	$ 723,164	$ 714,129	$ 695,257	$ 688,853	$ 723,164	$ 695,257
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance (in shares)	48,134	48,065	48,074	48,012	48,065	48,012
Balance	$ 481	$ 481	$ 481	$ 480	$ 481	$ 480
Issuance of common stock (in shares)	8	109		95
Issuance of common stock				$ 1
Repurchase of common stock (in shares)	(2)	(40)	(4)	(33)
Balance (in shares)	48,140	48,134	48,070	48,074	48,140	48,070
Balance	$ 481	$ 481	$ 481	$ 481	$ 481	$ 481
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	338,728	337,623	335,625	336,229	337,623	336,229
Issuance of common stock	1,675	3,179	737	635
Repurchase of common stock	(129)	(2,074)	(124)	(1,239)
Balance	340,274	338,728	336,238	335,625	340,274	336,238
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	374,920	392,053	352,747	362,512	392,053	362,512
Net income	16,996	(7,640)	14,805	(762)
Dividends paid on common stock	(9,507)	(9,493)	(9,014)	(9,003)
Balance	$ 382,409	$ 374,920	$ 358,538	$ 352,747	$ 382,409	$ 358,538