Accumulated Other Comprehensive Income (Schedule of Reclassifications) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	$ 550,305	$ 688,544	$ 1,398,709	$ 2,010,011
Benefit from income taxes	3,270	343,865	4,626	351,332
Net loss	(51,421)	(1,999,476)	(434,010)	(2,280,774)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivative Instruments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	76	3,598	397	6,806
Benefit from income taxes	0	(1,295)	(81)	(2,452)
Net loss	76	2,303	316	4,354
Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension, Postretirement and Other Postemployment Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service credits	2,510	2,083	7,854	6,250
Amortization of actuarial gains (losses), net	(4,753)	(3,266)	(6,010)	(11,200)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, before Tax	(2,243)	(1,183)	1,844	(4,950)
Benefit from income taxes	0	426	(481)	1,782
Net of tax	(2,243)	(757)	1,363	(3,168)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Benefit from income taxes	0	358	1,038	1,914
Net loss	632	(723)	(1,225)	(3,394)
Interest and investment income	$ 632	$ (1,081)	$ (2,263)	$ (5,308)