Investments (Credit Quality Indicators) (Details) - Commercial Mortgage and Agricultural Property Loans - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	$ 1,151,939	$ 1,652,476
0%-59.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	752,317	1,047,092
60%-69.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	275,537	387,784
70%-79.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	119,344	209,679
80% or Greater
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	4,741	7,921
Debt Service Coverage Ratio, Greater than or Equal to 1.2X
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	1,092,909	1,580,758
Debt Service Coverage Ratio, Greater than or Equal to 1.2X | 0%-59.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	732,473	1,004,751
Debt Service Coverage Ratio, Greater than or Equal to 1.2X | 60%-69.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	267,122	378,799
Debt Service Coverage Ratio, Greater than or Equal to 1.2X | 70%-79.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	88,811	197,208
Debt Service Coverage Ratio, Greater than or Equal to 1.2X | 80% or Greater
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	4,503	0
Debt Service Coverage Ratio, 1.0X to less than 1.2X
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	57,892	55,957
Debt Service Coverage Ratio, 1.0X to less than 1.2X | 0%-59.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	19,844	35,579
Debt Service Coverage Ratio, 1.0X to less than 1.2X | 60%-69.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	7,515	4,969
Debt Service Coverage Ratio, 1.0X to less than 1.2X | 70%-79.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	30,533	12,471
Debt Service Coverage Ratio, 1.0X to less than 1.2X | 80% or Greater
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	0	2,938
Debt Service Coverage Ratio, Less than 1.0X
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	1,138	15,761
Debt Service Coverage Ratio, Less than 1.0X | 0%-59.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	0	6,762
Debt Service Coverage Ratio, Less than 1.0X | 60%-69.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	900	4,016
Debt Service Coverage Ratio, Less than 1.0X | 70%-79.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	0	0
Debt Service Coverage Ratio, Less than 1.0X | 80% or Greater
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	$ 238	$ 4,983