Other Comprehensive Income Reclassifications from Accumulated Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Reclassification Out Of Accumulated Other Comprehensive Income [Line Items]
Prior-service costs	$ (17)		$ (17)		$ (34)		$ (34)
Actuarial losses	246		2		497		4
Income before income taxes	14,092		42,855		14,345		78,056
Tax benefit (provision)	(1,896)		(10,564)		1,347		(20,193)
Total, net of tax	12,196		32,291		15,692		57,863
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Out Of Accumulated Other Comprehensive Income [Line Items]
Total, net of tax	(183)	[1]	(22)	[1]	(369)	[1]	(45)	[1]
Accumulated Defined Benefit Plans Adjustment [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Out Of Accumulated Other Comprehensive Income [Line Items]
Prior-service costs	(17)	[1],[2]	(17)	[1],[2]	(34)	[1],[2]	(34)	[1],[2]
Actuarial losses	(246)	[1],[2]	(2)	[1],[2]	(497)	[1],[2]	(4)	[1],[2]
Income before income taxes	(263)	[1]	(19)	[1]	(531)	[1]	(38)	[1]
Tax benefit (provision)	80	[1]	(3)	[1]	162	[1]	(7)	[1]
Total, net of tax	$ (183)	[1]	$ (22)	[1]	$ (369)	[1]	$ (45)	[1]

[1] Amounts in parenthesis indicate debits to the Statements of Operations.

[2] These AOCI components are included in the computation of net periodic pension cost. Refer to Note 8 for additional details.