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 (in shares) at Dec. 31, 2016		47,965
Balance at the beginning of the period at Dec. 31, 2016	$ 659,471	$ 480	$ 334,856	$ 324,135
Increase (Decrease) in Stockholders' Equity
Net income	72,940			72,940
Issuance of common stock (in shares)		88
Issuance of common stock	2,878	$ 1	2,877
Repurchase of common stock (in shares)		(41)
Repurchase of common stock	(1,505)	$ (1)	(1,504)
Dividends paid on common stock	(34,563)			(34,563)
Balance at the beginning of the period (in shares) at Dec. 31, 2017		48,012
Balance at the end of the period at Dec. 31, 2017	699,221	$ 480	336,229	362,512
Increase (Decrease) in Stockholders' Equity
Net income	65,584			65,584
Issuance of common stock (in shares)		95
Issuance of common stock	3,040	$ 1	3,039
Repurchase of common stock (in shares)		(42)
Repurchase of common stock	(1,645)	$ 0	(1,645)
Dividends paid on common stock	(36,043)			(36,043)
Balance at the beginning of the period (in shares) at Dec. 31, 2018		48,065
Balance at the end of the period at Dec. 31, 2018	730,157	$ 481	337,623	392,053
Increase (Decrease) in Stockholders' Equity
Net income	63,116			63,116
Issuance of common stock (in shares)		515
Issuance of common stock	27,153	$ 5	27,148
Repurchase of common stock (in shares)		(48)
Repurchase of common stock	(2,497)	$ (1)	(2,496)
Dividends paid on common stock	(38,023)			(38,023)
Balance at the beginning of the period (in shares) at Dec. 31, 2019		48,532
Balance at the end of the period at Dec. 31, 2019	$ 779,906	$ 485	$ 362,275	$ 417,146