Consolidated Statements of Common Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings
Balance at the beginning of the period at Dec. 31, 2014	$ 626,626	$ 478	$ 330,558	$ 295,590
Balance (in shares) at Dec. 31, 2014		47,806
Increase (Decrease) in Stockholders' Equity
Net income	45,017			45,017
Issuance of common stock	2,578	$ 1	2,577
Issuance of common stock (in shares)		69
Dividends paid on common stock	(32,066)			(32,066)
Balance at the end of the period at Dec. 31, 2015	642,155	$ 479	333,135	308,541
Balance (in shares) at Dec. 31, 2015		47,875
Increase (Decrease) in Stockholders' Equity
Net income	48,675			48,675
Issuance of common stock	1,722	$ 1	1,721
Issuance of common stock (in shares)		90
Dividends paid on common stock	(33,081)			(33,081)
Balance at the end of the period at Dec. 31, 2016	659,471	$ 480	334,856	324,135
Balance (in shares) at Dec. 31, 2016		47,965
Increase (Decrease) in Stockholders' Equity
Net income	67,181			67,181
Issuance of common stock	1,373	$ 0	1,373
Issuance of common stock (in shares)		47
Dividends paid on common stock	(34,563)			(34,563)
Balance at the end of the period at Dec. 31, 2017	$ 693,462	$ 480	$ 336,229	$ 356,753
Balance (in shares) at Dec. 31, 2017		48,012