Accumulated Other Comprehensive Loss - Summary of Changes in Accumulated Other Comprehensive Loss for Pension Plan Items (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss beginning balance	$ (7,517)	$ (8,652)
Other comprehensive income before reclassifications	(2,689)	159
Amounts reclassified from accumulated other comprehensive loss	8,881	976
Total other comprehensive income, net of tax	6,192	1,135	1,228
Accumulated other comprehensive loss ending balance	(1,325)	(7,517)	(8,652)
Settlement cost	0	0	2,503
Investment income, net	12,691	1,498	707
Reclassification of realized losses (gains) included in earnings	833	(93)	0
Total before tax	521,821	4,445	6,423
Income tax (expense) benefit	(115,507)	409	(387)
Net income	406,282	4,966	5,990
Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net income	8,881	976
Defined Benefit Pension Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss beginning balance	(5,392)	(8,652)
Other comprehensive income before reclassifications	(2,180)	2,191
Amounts reclassified from accumulated other comprehensive loss	7,572	1,069
Total other comprehensive income, net of tax	5,392	3,260
Accumulated other comprehensive loss ending balance	0	(5,392)
Defined Benefit Pension Items | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Amortization of loss	465	326
Settlement cost	7,107	743
Total before tax	7,572	1,069
Income tax (expense) benefit	0	0
Net income	7,572	1,069
Other-than-temporary impairment losses | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Investment income, net	1,295	0
Total before tax	2,128	(93)
Unrealized Gains and Losses on Available-for-Sale Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss beginning balance	(2,125)	0
Other comprehensive income before reclassifications	(509)	(2,032)
Amounts reclassified from accumulated other comprehensive loss	1,309	(93)
Total other comprehensive income, net of tax	800	(2,125)
Accumulated other comprehensive loss ending balance	(1,325)	(2,125)
Unrealized Gains and Losses on Available-for-Sale Securities | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Reclassification of realized losses (gains) included in earnings	833	(93)
Income tax (expense) benefit	(819)	0
Net income	$ 1,309	$ (93)