Investments (Credit Quality Indicators) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	$ 1,080,864	$ 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	663,425	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	284,545	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	94,080	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	2,203	0
Greater than 1.2X | Total | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	1,044,253	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	11,944	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	7,990	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	12,319	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	0	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	32,253	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	4,119	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	239	4,983
Less than 1.0X | Total | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	4,358	15,761
Total | 0%-59.99% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	679,488	1,047,092
Total | 60%-69.99% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	292,535	387,784
Total | 70%-79.99% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	106,399	209,679
Total | Greater than 80% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	2,442	7,921
Total | Total | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	$ 1,080,864	$ 1,652,476