Investments (Credit Quality Indicators) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	$ 1,024,578	$ 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	696,210	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	208,923	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	94,894	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	1,000,027	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	14,627	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	2,403	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	5,307	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	2,214	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	24,551	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	0	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	0	15,761
Total | 0%-59.99% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	710,837	1,047,092
Total | 60%-69.99% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	211,326	387,784
Total | 70%-79.99% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	100,201	209,679
Total | Greater than 80% | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	2,214	7,921
Total | Total | Commercial Mortgage and Agricultural Loans
Credit Quality Indicators [Line Items]
Total commercial mortgage and agricultural property loans	$ 1,024,578	$ 1,652,476