Quarterly Financial Information (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2015

Quarterly Financial Information (Unaudited) [Abstract]

Schedule of Quarterly Financial Information [Table Text Block]

	First		Second		Third		Fourth
	(In Thousands, Except Per Share Data)
Year ended December 31, 2015
Operating revenue	$	187,523	$	191,684	$	182,533	$	174,605
Operating income	28,261		35,739		24,861		27,719
Income before income taxes	28,273		35,800		24,926		27,772
Net income	17,612		23,316		15,113		17,015
Net income per share, basic	0.20		0.27		0.17		0.20
Net income per share, diluted	0.20		0.27		0.17		0.20
Year ended December 31, 2014
Operating revenue	$	224,481	$	226,785	$	217,092	$	202,997
Operating income	20,688		40,642		36,290		34,247
Income before income taxes	20,569		40,616		36,214		34,216
Net income	14,079		26,472		22,737		21,544
Net income per share, basic	0.16		0.30		0.26		0.25
Net income per share, diluted	0.16		0.30		0.26		0.25