Accumulated Other Comprehensive Loss Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Schedule of Reclassifciations Out of Accumulated Other Comprehensive Income [Line Items]
Income (loss) before income taxes	$ 184,146		$ 149,513		$ 120,133
Income tax benefit	(61,501)		(53,811)		(44,568)
Net income	122,645		95,702		75,565
Reclassification out of Accumulated Other Comprehensive Income
Schedule of Reclassifciations Out of Accumulated Other Comprehensive Income [Line Items]
Net income	(339)	[1]	(339)	[1]
Defined Benefit Plan Items | Reclassification out of Accumulated Other Comprehensive Income
Schedule of Reclassifciations Out of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior-service cost	(543)	[1],[2]	(543)	[1],[2]
Income (loss) before income taxes	(543)	[1]	(543)	[1]
Income tax benefit	204	[1]	204	[1]
Net income	$ (339)	[1]	$ (339)	[1]

[1] Amounts in parentheses indicate debits to profit/loss

[2] This accumulated other comprehensive income component is included in the computation of net periodic benefit cost (see Note 6. Employee Benefit Plans footnote for additional details)