Schedule I - Condensed Financial Information Of Registrant (Unconsolidated Statements Of Cash Flows) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Interest Paid, Excluding Capitalized Interest, Operating Activities	$ 117,457	$ 90,975	$ 94,781
Net income	132,393	134,025	108,793
Amortization of debt issuance costs and discounts	(4,109)	(3,975)	(4,202)
Increase (Decrease) in Accounts Payable	3,047	3,675	(12,917)
Accrued and other current liabilities	4,413	(10,532)	97
Other - net	(7,357)	1,089	2,038
Net cash provided by operating activities	397,815	381,012	285,260
Payments for (Proceeds from) Other Investing Activities	278	1,053	323
Net cash used in investing activities	(237,448)	(253,952)	(236,432)
Long-term borrowings, net of discount	0	169,936	404,633
Retirement of long-term debt	0	0	(408,152)
Dividends on common stock	(136,426)	(130,179)	(105,144)
Other - net	152	190	228
Net cash used in financing activities	(145,414)	(124,142)	(53,100)
Cash and cash equivalents at beginning of period	33,199
Cash, cash equivalents and restricted cash at end of period	48,152	33,199
Income Taxes Paid, Net	29,600	28,275	65,050
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Interest Paid, Excluding Capitalized Interest, Operating Activities	28,911	29,665	31,750
Net income	129,180	130,812	105,580
Equity in earnings of subsidiaries	(154,078)	(154,150)	(133,725)
Cash dividends received from subsidiary companies	159,000	142,250	132,516
Amortization of debt issuance costs and discounts	(1,847)	(1,964)	(2,003)
Deferred income taxes - net	157	(89)	78
Charges related to early extinguishment of debt	0	0	8,875
Increase (Decrease) in Accounts Payable	231	(405)	(1,833)
Accrued and other current liabilities	(3)	(1,244)	7,413
Other - net	(886)	(1,838)	370
Net cash provided by operating activities	135,448	117,300	121,277
Investment in subsidiaries	0	(65,000)	0
Payments for (Proceeds from) Other Investing Activities	278	1,053	0
Net cash used in investing activities	278	(63,947)	0
Long-term borrowings, net of discount	0	65,000	404,633
Retirement of long-term debt	0	0	(408,152)
Dividends on common stock	(136,426)	(130,179)	(105,144)
Other - net	0	148	3,601
Net cash used in financing activities	(136,426)	(65,327)	(112,264)
Cash and Cash Equivalents, Period Increase (Decrease)	(700)	(11,974)	9,013
Cash and cash equivalents at beginning of period	4,409	16,383
Cash, cash equivalents and restricted cash at end of period	3,709	4,409	16,383
Income Taxes Paid, Net	$ 29,600	$ 28,275	$ 65,050