Selected Quarterly Financial Data (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 [Abstract]
Operating revenues	$ 226,042	$ 243,626	$ 218,892	$ 201,132	$ 205,747	$ 226,137	$ 211,860	$ 194,347	$ 889,692	$ 838,091	$ 809,525
Operations and maintenance expense	85,321	82,022	86,445	79,314	92,393	68,624	73,515	73,946	333,102	308,478	282,253
Operating income	85,301	106,470	80,949	67,439	62,794	104,293	86,754	69,337	340,159	323,178	333,920
Net income (loss)	$ 64,227	$ 88,489	$ 54,903	$ 16,924	$ (3,657)	$ 78,216	$ 66,590	$ 50,839	$ 224,543	$ 191,988	$ 239,738
Basic net income (loss) per common share	$ 0.28	$ 0.38	$ 0.25	$ 0.09	$ (0.02)	$ 0.44	$ 0.37	$ 0.29	$ 1.04	$ 1.08	$ 1.35
Diluted net income (loss) per common share	0.28	0.38	0.25	0.09	(0.02)	0.44	0.37	0.29	1.04	1.08	1.35
Dividend paid per common share	0.2343	0.2343	0.2190	0.2190	0.2190	0.2190	0.2047	0.2047	0.9066	0.8474
Dividend declared per common share	$ 0.2343	$ 0.2343	$ 0.2190	$ 0.2190	$ 0.2190	$ 0.2190	$ 0.2047	$ 0.2047	$ 0.9066	$ 0.8474	$ 0.7920