Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Quarterly results of operations

Quarterly results of operations for the years ended December 31, 2017 and 2016 are summarized below:

	Three Months Ended
	March 31		June 30		September 30		December 31
	(in thousands, except per share amounts)
2017
Operating revenue	$	591,746	$	701,682	$	687,231	$	667,007
Operating income	59,405		132,736		104,104		92,546
Net income (1)	31,935		78,143		60,190		250,338
Basic earnings per share (1)	0.46		1.13		0.87		3.64
Diluted earnings per share (1)	0.46		1.12		0.87		3.63
2016
Operating revenue	$	538,143	$	584,133	$	621,329	$	578,351
Operating income	101,299		121,835		135,216		85,311
Net income	61,920		73,084		81,382		48,493
Basic earnings per share	0.87		1.03		1.17		0.70
Diluted earnings per share	0.86		1.03		1.17		0.70