Pension and Other Postretirement Benefit Plans (Schedule of Changes in Benefit Obligation, Plan Assets and Funded Status) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Change in plan assets:
Fair value of plan assets at beginning of period	$ 679,245,000
Fair value of plan assets at end of period	578,347,000	$ 679,245,000
Pension Benefits
Change in benefit obligation:
Benefit obligation at beginning of period	733,990,000	735,177,000
Service cost	1,150,000	2,948,000	$ 1,533,000
Interest cost	23,505,000	25,265,000	25,048,000
Actuarial (gain) loss	(60,351,000)	35,281,000
Plan settlements	0	(29,142,000)
Benefits and other payments	(54,152,000)	(35,539,000)
Benefit obligation at end of period	644,142,000	733,990,000	735,177,000
Change in plan assets:
Fair value of plan assets at beginning of period	679,245,000	632,434,000
Actual return on plan assets	(48,470,000)	110,311,000
Company contributions	1,724,000	1,181,000
Benefits and other payments	(54,152,000)	(35,539,000)
Plan settlements	0	(29,142,000)
Fair value of plan assets at end of period	578,347,000	679,245,000	632,434,000
Funded status:
Current liabilities	(1,623,000)	(1,785,000)
Noncurrent liabilities	(64,172,000)	(52,960,000)
Net obligation recognized	(65,795,000)	(54,745,000)
Amounts recognized in accumulated other comprehensive income consist of:
Net actuarial loss	263,229,000	243,456,000
Prior service credit	(367,000)	(869,000)
Net amount recognized (before tax effect)	262,862,000	242,587,000
Other Postretirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of period	591,563,000	700,085,000
Service cost	0	0	0
Interest cost	18,706,000	23,945,000	24,241,000
Actuarial (gain) loss	(101,259,000)	(101,379,000)
Plan settlements	0	0
Benefits and other payments	(35,419,000)	(31,088,000)
Benefit obligation at end of period	473,591,000	591,563,000	700,085,000
Change in plan assets:
Fair value of plan assets at beginning of period	0	0
Actual return on plan assets	0	0
Company contributions	35,419,000	31,088,000
Benefits and other payments	(35,419,000)	(31,088,000)
Plan settlements	0	0
Fair value of plan assets at end of period	0	0	$ 0
Funded status:
Current liabilities	(32,345,000)	(37,464,000)
Noncurrent liabilities	(441,246,000)	(554,099,000)
Net obligation recognized	(473,591,000)	(591,563,000)
Amounts recognized in accumulated other comprehensive income consist of:
Net actuarial loss	184,438,000	301,901,000
Prior service credit	(23,354,000)	(25,759,000)
Net amount recognized (before tax effect)	$ 161,084,000	$ 276,142,000