Accumulated Other Comprehensive Income (Schedule of Reclassifications) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	$ 420,298	$ 644,462	$ 848,404	$ 1,321,467
Benefit from income taxes	245	4,071	1,356	7,467
Net loss	(175,887)	(168,103)	(382,589)	(281,298)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivative Instruments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	96	2,727	321	3,208
Benefit from income taxes	0	(983)	(81)	(1,157)
Net loss	96	1,744	240	2,051
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	2,083	5,344	4,167
Amortization of actuarial gains (losses), net	77	(5,556)	(1,257)	(7,934)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, before Tax	2,749	(3,473)	4,087	(3,767)
Benefit from income taxes	0	1,251	(481)	1,356
Net of tax	2,749	(2,222)	3,606	(2,411)
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	549	1,038	1,556
Net loss	0	(881)	(1,857)	(2,671)
Interest and investment income	$ 0	$ (1,430)	$ (2,895)	$ (4,227)