Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Quarterly Financial Information [Abstract]
Operating revenues	$ 240,422	$ 88,190	$ 136,238	$ 297,323	$ 233,528	$ 87,727	$ 99,183	$ 255,529	$ 762,173	$ 675,967	$ 723,791
Net income (loss)	$ (90,167)	$ (8,495)	$ 2,729	$ 40,310	$ 28,275	$ (8,040)	$ 2,019	$ 36,641	$ (55,623)	$ 58,895	$ 53,703
Earnings (loss) per share of common stock - basic	$ (3.14)	$ (0.30)	$ 0.10	$ 1.41	$ 1.01	$ (0.29)	$ 0.07	$ 1.33	$ (1.94)	$ 2.13	$ 1.96
Earnings (loss) per share of common stock - diluted	$ (3.14)	$ (0.30)	$ 0.10	$ 1.40	$ 1.00	$ (0.29)	$ 0.07	$ 1.33	$ (1.94)	$ 2.12	$ 1.96