Accumulated Other Comprehensive Income (Schedule of Reclassifications) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	$ 428,106	$ 677,005
Benefit from income taxes	1,111	3,396
Net loss	(206,702)	(113,195)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivative Instruments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	226	481
Benefit from income taxes	(82)	(174)
Net loss	144	307
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	2,672	2,084
Amortization of actuarial gains (losses), net	(1,334)	(2,378)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, before Tax	1,338	(294)
Benefit from income taxes	(481)	105
Net of tax	857	(189)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Benefit from income taxes	1,038	1,007
Net loss	(1,857)	(1,790)
Interest and investment income	$ (2,895)	$ (2,797)