Investments (Credit Quality Indicators) (Details) - Commercial Mortgage and Agricultural Loans - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	$ 1,319,007	$ 1,356,374
0%-59.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	704,396	724,501
60%-69.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	452,839	465,568
70%-79.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	160,763	163,285
80% or greater
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	1,009	3,020
≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	1,273,681	1,309,699
≥ 1.2X | 0%-59.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	688,758	709,342
≥ 1.2X | 60%-69.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	431,303	442,308
≥ 1.2X | 70%-79.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	153,620	156,049
≥ 1.2X | 80% or greater
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	0	2,000
1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	40,696	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	12,017	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	21,536	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	7,143	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	0	1,020
Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	4,630	345
Less than 1.0X | 0%-59.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	3,621	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	$ 1,009	$ 0