Accumulated Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance as of January 1, 2013	$ 95,188
Change in accumulated other comprehensive income before reclassifications	9,855	8,842
Reclassification adjustments from accumulated other comprehensive income	(433)	(1,399)
Balance as of March 31, 2013	104,610
Net unrealized appreciation on investments [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance as of January 1, 2013	144,096
Change in accumulated other comprehensive income before reclassifications	9,855
Reclassification adjustments from accumulated other comprehensive income	(1,241)
Balance as of March 31, 2013	152,710
Liability for underfunded employee benefit costs [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance as of January 1, 2013	(48,908)
Change in accumulated other comprehensive income before reclassifications	0
Reclassification adjustments from accumulated other comprehensive income	808
Balance as of March 31, 2013	$ (48,100)