Accumulated Other Comprehensive Income (Schedule of Reclassifications) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	$ 633,180	$ 613,538	$ 1,800,824	$ 1,764,379
Interest expense	(5,179)	(5,972)	(15,624)	(21,400)
Pension settlement	(971)	(821)	(2,206)	(1,774)
Interest and investment income	1,801	720	4,626	2,089
Net income	123,192	68,351	226,483	157,179
Reclassification out of Accumulated Other Comprehensive Income | Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	(4,824)	(88)	(4,824)	(88)
Interest expense	265	0	753	0
Net income	(4,559)	(88)	(4,071)	(88)
Reclassification out of Accumulated Other Comprehensive Income | Pension settlement
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pension settlement	613	228	1,984	715
Reclassification out of Accumulated Other Comprehensive Income | Actuarial curtailments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pension settlement	0	(773)	0	(773)
Reclassification out of Accumulated Other Comprehensive Income | Sale of Cumberland River Pension Plan
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pension settlement	0	(360)	0	(360)
Reclassification out of Accumulated Other Comprehensive Income | Pension, postretirement and other post-employment benefits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net income	613	(905)	1,984	(418)
Reclassification out of Accumulated Other Comprehensive Income | Available-for-sale securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and investment income	8	0	24	332
Net income	$ 8	$ 0	$ 24	$ 332