Selected Quarterly Financial Data (Unaudited) Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Selected Quarterly Financial Data
Operating revenues	$ 190,103	[1]	$ 294,453	[1]	$ 203,075	[1]	$ 174,363	[1]	$ 190,823	[1]	$ 300,271	[1]	$ 236,796	[1]	$ 175,713	[1]	$ 861,994	$ 903,603	$ 916,797
Operating income	13,419		114,027		46,338		4,309		15,113		99,933		53,139		4,044		178,093	172,229	190,059
Net income (loss)	$ 12,942		$ 77,880		$ 26,126		$ 6,089		$ (15,285)		$ 73,271		$ 33,295		$ (6,966)		$ 123,037	$ 84,315	$ 98,261
Basic earnings per share:
Net income (loss)	$ 0.32		$ 1.91		$ 0.64		$ 0.15		$ (0.38)		$ 1.80		$ 0.82		$ (0.17)		$ 3.02	$ 2.070	$ 2.420
Diluted earnings per share:
Net income (loss)	0.32		1.91		0.64		0.15		(0.38)		1.79		0.82		(0.17)		3.01	2.070	2.420
Dividends declared per share of common stock	$ 0.385		$ 0.385		$ 0.385		$ 0.360		$ 0.360		$ 0.360		$ 0.360		$ 0.335		$ 1.515	$ 1.415	$ 1.315

[1] Operating revenues are seasonal in nature, with the peak sales periods generally occurring during the summer months. Comparisons among quarters of a year may not represent overall trends and changes in operations.