CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Parent [Line Items]
Net income (loss)	$ 146,897	$ 127,550
Other comprehensive income (loss):
Net unrealized gain (loss) from pension and postretirement plans, net of tax	227	880
Other Comprehensive (Income) Loss, Reclassification of Stranded Taxes to Retained Earnings	(5,230)	0
Other comprehensive income (loss)	(5,003)	880
Comprehensive income (loss)	141,894	128,430
Subsidiaries [Member]
Parent [Line Items]
Net income (loss)	163,037	143,092
Other comprehensive income (loss):
Net unrealized gain (loss) from pension and postretirement plans, net of tax	2,863	3,216
Amortization of treasury interest rate swaps to earnings, net of tax	96	79
Other Comprehensive (Income) Loss, Reclassification of Stranded Taxes to Retained Earnings	(27,333)	0
Other comprehensive income (loss)	(24,374)	3,295
Comprehensive income (loss)	$ 138,663	$ 146,387