Investments (Credit Quality Indicators) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	$ 1,644,032	$ 1,660,886
Greater than 1.2X | 0%-59.99% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	978,771	1,004,751
Greater than 1.2X | 60%-69.99% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	396,369	378,799
Greater than 1.2X | 70%-79.99% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	204,064	197,208
Greater than 1.2X | Greater than 80% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	0	0
Greater than 1.2X | Total | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	1,579,204	1,580,758
1.0X to 1.2X | 0%-59.99% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	22,821	35,579
1.0X to 1.2X | 60%-69.99% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	16,769	4,969
1.0X to 1.2X | 70%-79.99% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	13,617	12,471
1.0X to 1.2X | Greater than 80% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	2,924	2,938
1.0X to 1.2X | Total | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	56,131	55,957
Less than 1.0X | 0%-59.99% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	287	6,762
Less than 1.0X | 60%-69.99% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	0	4,016
Less than 1.0X | 70%-79.99% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	0	0
Less than 1.0X | Greater than 80% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	0	4,983
Less than 1.0X | Total | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	287	15,761
Total | 0%-59.99% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	1,001,879	1,047,092
Total | 60%-69.99% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	413,138	387,784
Total | 70%-79.99% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	217,681	209,679
Total | Greater than 80% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	2,924	7,921
Total | Total | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	$ 1,635,622	$ 1,652,476