Quarterly Results of Operations (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Schedule of Quarterly Results of Operations

(In thousands, except per share amounts)	First Quarter		Second Quarter		Third Quarter		Fourth Quarter
2017:
Operating revenues	$	501,221	$	519,508	$	528,643	$	567,365
Operating income	25,972		36,913		35,874		45,061
Net income	16,019		23,219		22,517		141,134
Basic earnings per share	0.22		0.32		0.31		1.95
Diluted earnings per share	0.22		0.32		0.31		1.94

(In thousands, except per share amounts)	First Quarter		Second Quarter		Third Quarter		Fourth Quarter
2016:
Operating revenues	$	482,802	$	498,681	$	508,676	$	518,832
Operating income	32,487		29,553		29,074		34,956
Net income	20,092		18,306		18,920		21,811
Basic earnings per share	0.28		0.25		0.26		0.30
Diluted earnings per share	0.28		0.25		0.26		0.30