ACCUMULATED OTHER COMPREHENSIVE INCOME/(LOSS) - Schedule of Amounts Reclassified From AOCI By Component (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Pension SERP And PBOP Gain Loss To Be Reclassified Within Twelve Months									$ (6,600)
Tax Effect									(554,997)	$ (539,967)	$ (468,297)
Net Income Attributable to Common Shareholders	$ 229,200	$ 265,300	$ 203,600	$ 244,200	$ 181,800	$ 235,900	$ 207,500	$ 253,300	942,302	878,485	819,546
Defined Benefit Plan Costs:
Total Amounts Reclassified from AOCL, Net of Tax									(6,000)	(6,300)	(5,900)
Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax Effect									1,400	1,400	1,400
Qualified Cash Flow Hedging Instruments [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Income Attributable to Common Shareholders									(2,100)	(2,100)	(2,000)
Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent [Member]
Defined Benefit Plan Costs:
Amortization of Actuarial Losses									(5,600)	6,600	6,200
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent [Member]
Defined Benefit Plan Costs:
Amortization of Actuarial Losses									(800)	(200)	(200)
Defined Benefit Plans [Member]
Defined Benefit Plan Costs:
Amortization of Actuarial Losses									(6,400)	(6,800)	(6,400)
Tax Effect									2,500	2,600	2,500
Total Amounts Reclassified from AOCL, Net of Tax									(3,900)	(4,200)	(3,900)
Interest Rate Contract [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Qualified Cash Flow Hedging Instruments									$ (3,500)	$ (3,500)	$ (3,400)