Investments (Details 2) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014
Investments
Fair Value	$ 3,191,539
Cost or Amortized Cost	2,880,511
Pre-refunded bonds
Investments
Fair Value	$ 16,607	$ 8,367
Credit concentration \| Corporate fixed maturity and equity securities \| Investment rated as investment grade \| Energy Sector
Investments
Concentration percentage	95.00%	96.00%
Investment portfolio \| Credit concentration \| Corporate fixed maturity and equity securities \| Energy Sector
Investments
Cost or Amortized Cost	$ 253,847	$ 314,978
Gross Unrealized Gains	9,743	43,072
Investment portfolio \| Credit concentration \| Corporate fixed maturity and equity securities \| Europe
Investments
Cost or Amortized Cost	300,740	343,181
Gross Unrealized Gains	$ 33,752	$ 52,435
Investment portfolio \| Credit concentration \| Corporate fixed maturity and equity securities \| Other individual European countries \| Maximum
Investments
Concentration percentage	27.00%	26.00%
Investment portfolio \| Credit concentration \| States, municipalities and political subdivisions \| Municipal revenue bonds
Investments
Concentration percentage	31.00%	39.00%
Investment portfolio \| Credit concentration \| States, municipalities and political subdivisions \| Municipal revenue bonds \| Individual U. S. states \| Maximum
Investments
Concentration percentage	0.50%	0.50%
Investment portfolio \| Credit concentration \| States, municipalities and political subdivisions \| Corporate fixed maturity and equity securities \| United Kingdom
Investments
Concentration percentage	5.00%	6.00%

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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