Pension and Other Postretirement Benefit Plans (Schedule of Changes in Benefit Obligation, Plan Assets and Funded Status) (Details) - USD ($)			12 Months Ended
	Aug. 31, 2015	May 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in plan assets:
Fair value of plan assets at beginning of period			$ 632,434,000
Fair value of plan assets at end of period			679,245,000	$ 632,434,000
Pension Benefits
Change in benefit obligation:
Benefit obligation at beginning of period			735,177,000	751,617,000
Service cost			2,948,000	1,533,000	$ 8,256,000
Interest cost			25,265,000	25,048,000	31,655,000
Actuarial loss (gain)			35,281,000	46,885,000
Plan amendments	$ (17,793,000)		0	0
Plan settlements			(29,142,000)	(54,197,000)
Benefits and other payments			(35,539,000)	(35,709,000)
Benefit obligation at end of period			733,990,000	735,177,000	751,617,000
Change in plan assets:
Fair value of plan assets at beginning of period			632,434,000	669,039,000
Actual return on plan assets			110,311,000	50,575,000
Company contributions			1,181,000	2,726,000
Benefits and other payments			(35,539,000)	(35,709,000)
Plan settlements			(29,142,000)	(54,197,000)
Fair value of plan assets at end of period			679,245,000	632,434,000	669,039,000
Funded status:
Current liabilities			(1,785,000)	(2,871,000)
Noncurrent liabilities			(52,960,000)	(99,872,000)
Net obligation recognized			(54,745,000)	(102,743,000)
Amounts recognized in accumulated other comprehensive income consist of:
Net actuarial loss			243,456,000	295,152,000
Prior service credit			(869,000)	(1,372,000)
Net amount recognized (before tax effect)			242,587,000	293,780,000
Other Postretirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of period			700,085,000	671,755,000
Service cost			0	0	0
Interest cost			23,945,000	24,241,000	27,238,000
Actuarial loss (gain)			(101,379,000)	77,640,000
Plan amendments		$ 43,598,000	0	28,164,000
Plan settlements			0	0
Benefits and other payments			(31,088,000)	(45,387,000)
Benefit obligation at end of period			591,563,000	700,085,000	671,755,000
Change in plan assets:
Fair value of plan assets at beginning of period			0	0
Actual return on plan assets			0	0
Company contributions			31,088,000	45,387,000
Benefits and other payments			(31,088,000)	(45,387,000)
Plan settlements			0	0
Fair value of plan assets at end of period			0	0	$ 0
Funded status:
Current liabilities			(37,464,000)	(40,611,000)
Noncurrent liabilities			(554,099,000)	(659,474,000)
Net obligation recognized			(591,563,000)	(700,085,000)
Amounts recognized in accumulated other comprehensive income consist of:
Net actuarial loss			301,901,000	426,392,000
Prior service credit			(25,759,000)	(28,164,000)
Net amount recognized (before tax effect)			$ 276,142,000	$ 398,228,000