Summary of Quarterly Operating Results (unaudited) (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Summary of Quarterly Operating Results (Unaudited)

(Amounts in thousands, except per share data)

Three months ended, 2017	March 31		June 30		September 30		December 31
Operating revenue	$	317,037	$	358,160	$	350,062	$	353,251
Operating income		17,519		29,718		24,602		22,898
Net income		11,387		17,603		14,407		47,759
Basic earnings per share	$	0.45	$	0.69	$	0.56	$	1.87
Diluted earnings per share	$	0.44	$	0.68	$	0.55	$	1.82
Three months ended, 2016	March 31		June 30		September 30		December 31
Operating revenue	$	289,911	$	311,905	$	316,442	$	300,223
Operating income		17,584		21,721		22,644		17,187
Net income		10,575		13,275		13,826		10,348
Basic earnings per share	$	0.42	$	0.53	$	0.55	$	0.41
Diluted earnings per share	$	0.42	$	0.52	$	0.54	$	0.40