Accumulated Other Comprehensive Income (Loss) - Summary of Changes in Accumulated Other Comprehensive Loss for Pension Plan Items (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss, beginning balance	$ 648	$ (7,115)	$ (1,325)	$ (7,517)
Other comprehensive income before reclassifications	2,245	(1,911)	4,218	(2,451)
Amounts reclassified from accumulated other comprehensive income (loss)	(2,754)	1,192	(2,754)	2,134
Total other comprehensive (loss) income, net of tax	(509)	(719)	1,464	(317)
Accumulated other comprehensive loss, beginning balance	139	(7,834)	139	(7,834)
Investment income, net	9,125	3,367	19,776	7,592
Income (loss) before equity in loss from unconsolidated affiliates and income taxes	4,002	(531)	2,805	532,682
Income tax	(1,244)	386	(2,034)	(115,209)
Net income (loss)	2,758	(156)	771	417,441
Amount Reclassified from Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net income (loss)	(2,754)	1,192	(2,754)	2,134
Defined Benefit Pension Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss, beginning balance	0	(5,330)	0	(5,392)
Other comprehensive income before reclassifications	0	58	0	(309)
Amounts reclassified from accumulated other comprehensive income (loss)	0	396	0	825
Total other comprehensive (loss) income, net of tax	0	454	0	516
Accumulated other comprehensive loss, beginning balance	0	(4,876)	0	(4,876)
Defined Benefit Pension Items | Amount Reclassified from Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Amortization of loss	0	115	0	373
Settlement cost	0	529	0	969
Income (loss) before equity in loss from unconsolidated affiliates and income taxes	0	644	0	1,342
Income tax	0	(248)	0	(517)
Net income (loss)	0	396	0	825
Other-than-temporary impairment losses | Amount Reclassified from Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Investment income, net	0	1,295	0	1,295
Realized (gain) loss on sale
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss, beginning balance	648	(1,785)	(1,325)	(2,125)
Other comprehensive income before reclassifications	2,245	(1,969)	4,218	(2,142)
Amounts reclassified from accumulated other comprehensive income (loss)	(2,754)	796	(2,754)	1,309
Total other comprehensive (loss) income, net of tax	(509)	(1,173)	1,464	(833)
Accumulated other comprehensive loss, beginning balance	139	(2,958)	139	(2,958)
Realized (gain) loss on sale | Amount Reclassified from Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Investment income, net	(5,276)	0	(5,276)	833
Income tax	2,522	(499)	2,522	(819)
Net income (loss)	$ (2,754)	$ 796	$ (2,754)	$ 1,309