Quarterly Results of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 620,302	$ 590,214	$ 568,959	$ 592,703	$ 621,787	$ 618,264	$ 627,533	$ 596,117	$ 2,372,178	$ 2,463,701	$ 2,457,914
Operating income	81,451	62,103	52,818	31,066	65,654	53,357	58,442	48,019	227,438	225,472	224,215
Net income	$ 60,556	$ 46,332	$ 39,132	$ 23,058	$ 48,496	$ 39,044	$ 43,318	$ 36,086	$ 169,078	$ 166,944	$ 168,148
Basic earnings per share	$ 0.88	$ 0.67	$ 0.57	$ 0.33	$ 0.70	$ 0.56	$ 0.62	$ 0.51	$ 2.45	$ 2.40	$ 2.35
Diluted earnings per share	$ 0.88	$ 0.67	$ 0.56	$ 0.33	$ 0.70	$ 0.56	$ 0.62	$ 0.51	$ 2.44	$ 2.38	$ 2.33