Accumulated Other Comprehensive Loss - Summary of Changes in Accumulated Other Comprehensive Loss for Pension Plan Items (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss, beginning balance	$ (1,325)	$ (7,517)
Other comprehensive income before reclassifications	773	(736)
Amounts reclassified from accumulated other comprehensive loss		476
Total other comprehensive income (loss), net of tax	773	(260)
Accumulated other comprehensive loss, beginning balance	(552)	(7,777)
Realized loss on sale	5,212	274
(Loss) income before equity in loss from unconsolidated affiliates and income taxes	(2,789)	509,904
Income tax benefit	1,073	(106,905)
Net (loss) income	(1,716)	402,989
Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net (loss) income	0	476
Defined benefit pension items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss, beginning balance	0	(5,392)
Other comprehensive income before reclassifications	0	(252)
Amounts reclassified from accumulated other comprehensive loss		229
Total other comprehensive income (loss), net of tax		(23)
Accumulated other comprehensive loss, beginning balance	0	(5,415)
Defined benefit pension items | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Amortization of loss	0	132
Settlement cost	0	240
(Loss) income before equity in loss from unconsolidated affiliates and income taxes	0	372
Income tax benefit	0	143
Net (loss) income	0	229
Items related to available-for-sale securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss, beginning balance	(1,325)	(2,125)
Other comprehensive income before reclassifications	773	(484)
Amounts reclassified from accumulated other comprehensive loss		247
Total other comprehensive income (loss), net of tax		(237)
Accumulated other comprehensive loss, beginning balance	(552)	(2,362)
Items related to available-for-sale securities | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Realized loss on sale	0	401
Income tax benefit	0	154
Net (loss) income	$ 0	$ 247