CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net Income (Loss) Attributable to Parent	$ 210,708,000	$ 235,622,000	$ 175,194,000
Other comprehensive income (loss):
Net unrealized gain (loss) from pension and postretirement plans, net of tax	(6,947,000)	(47,690,000)	9,430,000
Reclassification of stranded taxes to retained earnings due to tax reform	0	(5,230,000)	0
Other comprehensive income (loss)	(6,947,000)	(52,920,000)	9,430,000
Comprehensive income (loss)	203,761,000	182,702,000	184,624,000
Subsidiaries [Member]
Net Income (Loss) Attributable to Parent	292,924,000	317,162,000	320,054,000
Other comprehensive income (loss):
Net unrealized gain (loss) from pension and postretirement plans, net of tax	2,022,000	(37,030,000)	18,288,000
Amortization of treasury interest rate swaps to earnings, net of tax	385,000	385,000	317,000
Reclassification of stranded taxes to retained earnings due to tax reform	0	27,333,000	0
Other comprehensive income (loss)	2,407,000	(63,978,000)	18,605,000
Comprehensive income (loss)	$ 295,331,000	$ 253,184,000	$ 338,659,000