Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Operating revenues	$ 425,283	$ 347,341	$ 1,477,837	$ 1,372,316	$ 1,198,670
Operating income	83,228	79,990	263,079	275,681	236,204
Net income	$ 66,743	$ 51,336	$ 183,008	$ 186,840	$ 155,215
Average Common Shares Outstanding	58,345,000	53,293,000	55,769,156	51,709,229	50,559,208
Income per average common share
Income per average common share, basic	$ 1.16	$ 0.96	$ 3.28	$ 3.61	$ 3.07
Income per average common share, diluted	$ 1.16	$ 0.96	$ 3.25	$ 3.60	$ 3.06