Schedule I - Condensed Financial Information Of Registrant (Unconsolidated Statements Of Income) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Long-term Debt, Excluding Current Maturities	$ 2,477,538	$ 2,474,840
Loss on early extinguishment of debt	(8,875)	0	$ (21,956)
Income taxes - net	48,951	61,209	31,566
Interest on long-term debt	(117,162)	(111,611)	(106,936)
Amortization of redemption premiums and expense on debt	(4,202)	(4,147)	(5,067)
NET INCOME	108,793	131,060	59,524
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Long-term Debt, Excluding Current Maturities	801,603	799,709
Equity in earnings of subsidiaries	133,725	153,232	98,708
Loss on early extinguishment of debt	(8,875)	0	(21,956)
Income taxes - net	16,495	11,483	24,650
Interest on long-term debt	33,787	33,973	41,659
Amortization of redemption premiums and expense on debt	(2,003)	(1,947)	(2,459)
Other -net	(25)	948	973
NET INCOME	$ 105,580	$ 127,847	$ 56,311