SCHEDULE I - CONDENSED FINANCIAL INFORMATION OF REGISTRANT - STATEMENTS OF INCOME AND CHANGES IN SHAREHOLDERS' EQUITY (Details) - USD ($) $ 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
Revenues
Revenues									$ 2,622,025	$ 2,614,831
Expenses:
Depreciation of property, plant and equipment									229,858	214,036	$ 200,658
Total expenses									2,525,927	2,527,493	2,209,392
Retirement defined benefits expense—other than service costs									2,806	5,962	7,942
Interest expense									109,339	104,216	91,128
Income before income taxes									271,409	254,461	276,580
Income benefits									(51,637)	(50,797)	(109,393)
Net income (loss)	$ 66,736	$ 63,890	$ 42,985	$ 46,161	$ 50,046	$ 66,371	$ 46,527	$ 40,720	219,772	203,664	167,187
Parent Company
Revenues
Revenues									777	429	798
Equity in net income of subsidiaries									246,005	226,972	187,097
Expenses:
Operating, administrative and general									19,195	19,515	16,578
Depreciation of property, plant and equipment									570	597	548
Taxes, other than income taxes									570	509	496
Total expenses									20,335	20,621	17,622
Income before interest expense and income (taxes) benefits									226,447	206,780	170,273
Retirement defined benefits expense—other than service costs									442	674	1,119
Interest expense									17,930	12,664	9,389
Income before income taxes									208,075	193,442	159,765
Income benefits									9,807	8,332	5,532
Net income (loss)									$ 217,882	$ 201,774	$ 165,297