Investments (Credit Quality Indicators) (Details) - Commercial Mortgage and Agricultural Loans - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	$ 1,337,506	$ 1,356,374
0%-59.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	646,323	724,501
60%-69.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	478,748	465,568
70%-79.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	210,254	163,285
80% or greater
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	2,181	3,020
≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	1,281,865	1,309,699
≥ 1.2X | 0%-59.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	635,907	709,342
≥ 1.2X | 60%-69.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	465,223	442,308
≥ 1.2X | 70%-79.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	179,558	156,049
≥ 1.2X | 80% or greater
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	1,177	2,000
1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	52,026	46,330
1.0X to 1.2X | 0%-59.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	6,801	14,814
1.0X to 1.2X | 60%-69.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	13,525	23,260
1.0X to 1.2X | 70%-79.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	30,696	7,236
1.0X to 1.2X | 80% or greater
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	1,004	1,020
Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	3,615	345
Less than 1.0X | 0%-59.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	3,615	345
Less than 1.0X | 60%-69.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	0	0
Less than 1.0X | 70%-79.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	0	0
Less than 1.0X | 80% or greater
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	$ 0	$ 0