Pension and Other Postretirement Benefit Plans (Schedule of Changes in Benefit Obligation, Plan Assets and Funded Status) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Change in plan assets:
Fair value of plan assets at beginning of period	$ 578,347,000
Fair value of plan assets at end of period	668,481,000	$ 578,347,000
Pension Benefits
Change in benefit obligation:
Benefit obligation at beginning of period	644,142,000	733,990,000
Service cost	3,950,000	1,150,000	$ 2,948,000
Interest cost	25,101,000	23,505,000	25,265,000
Actuarial loss (gain)	95,078,000	(60,351,000)
Benefits and other payments	(48,173,000)	(54,152,000)
Benefit obligation at end of period	720,098,000	644,142,000	733,990,000
Change in plan assets:
Fair value of plan assets at beginning of period	578,347,000	679,245,000
Actual return on plan assets	136,976,000	(48,470,000)
Company contributions	1,331,000	1,724,000
Benefits and other payments	(48,173,000)	(54,152,000)
Fair value of plan assets at end of period	668,481,000	578,347,000	679,245,000
Funded status:
Current liabilities	(1,687,000)	(1,623,000)
Noncurrent liabilities	(49,930,000)	(64,172,000)
Net obligation recognized	(51,617,000)	(65,795,000)
Amounts recognized in accumulated other comprehensive loss consist of:
Net actuarial loss	255,830,000	263,229,000
Prior service credit	0	(367,000)
Net amount recognized (before tax effect)	255,830,000	262,862,000
Other Postretirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of period	473,591,000	591,563,000
Service cost	0	0	0
Interest cost	18,320,000	18,706,000	23,945,000
Actuarial loss (gain)	4,761,000	(101,259,000)
Benefits and other payments	(32,343,000)	(35,419,000)
Benefit obligation at end of period	464,329,000	473,591,000	591,563,000
Change in plan assets:
Fair value of plan assets at beginning of period	0	0
Actual return on plan assets	0	0
Company contributions	32,343,000	35,419,000
Benefits and other payments	(32,343,000)	(35,419,000)
Fair value of plan assets at end of period	0	0	$ 0
Funded status:
Current liabilities	(31,833,000)	(32,345,000)
Noncurrent liabilities	(432,496,000)	(441,246,000)
Net obligation recognized	(464,329,000)	(473,591,000)
Amounts recognized in accumulated other comprehensive loss consist of:
Net actuarial loss	179,937,000	184,438,000
Prior service credit	(20,949,000)	(23,354,000)
Net amount recognized (before tax effect)	$ 158,988,000	$ 161,084,000