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
Operating revenues	$ 328,135	$ 274,836	$ 270,719	$ 384,220	$ 308,816	$ 279,874	$ 261,817	$ 341,502	$ 1,257,910	$ 1,192,009	$ 1,305,652
Operating income	84,651	46,356	48,823	97,020	64,742	47,808	69,210	84,512	276,850	266,272	271,749
Net income	$ 59,982	$ 21,670	$ 47,662	$ 72,806	$ 66,492	$ 28,182	$ 43,787	$ 58,499	$ 202,120	$ 196,960	$ 162,703
Average Common Shares Outstanding	50,448,000	50,444,000	50,441,000	50,381,000	50,321,000	50,318,000	49,869,000	49,416,000	50,428,560	49,984,562	48,557,599
Income per average common share
Income per average common share, basic	$ 1.19	$ 0.43	$ 0.94	$ 1.45	$ 1.32	$ 0.56	$ 0.88	$ 1.18	$ 4.01	$ 3.94	$ 3.35
Income per average common share, diluted	$ 1.18	$ 0.42	$ 0.94	$ 1.44	$ 1.31	$ 0.56	$ 0.87	$ 1.18	$ 3.98	$ 3.92	$ 3.34