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, 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	72,940			72,940
Issuance of common stock	1,373		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	699,221	$ 480	336,229	362,512
Balance (in shares) at Dec. 31, 2017		48,012
Increase (Decrease) in Stockholders' Equity
Net income	65,584			65,584
Issuance of common stock	1,395	$ 1	1,394
Issuance of common stock (in shares)		53
Dividends paid on common stock	(36,043)			(36,043)
Balance at the end of the period at Dec. 31, 2018	$ 730,157	$ 481	$ 337,623	$ 392,053
Balance (in shares) at Dec. 31, 2018		48,065