Income Taxes - Deferred tax (liabilities) assets (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Temporary differences [Abstract]
Property, plant and equipment	$ (42,418)	$ (44,448)
Goodwill and other intangible assets	(9,803)	(2,684)
Investments, primarily gains and losses not yet recognized for tax purposes	7,162	4,750
Accrued expenses not deductible until paid	13,011	11,865
Deferred compensation and retiree benefits not deductible until paid	66,002	37,041
Other temporary differences, net	4,135	25
Total temporary differences	38,089	6,549
Federal and state net operating loss carryforwards	13,605	21,123
Valuation allowance for state deferred tax assets	(912)	(1,078)
Deferred Tax Assets, Net	$ 50,782	$ 26,594