Investments (Credit Quality Indicators) (Details) - Commercial Mortgage and Agricultural Loans - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	$ 1,271,081	$ 1,356,374
0%-59.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	689,380	724,501
60%-69.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	411,251	465,568
70%-79.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	167,436	163,285
80% or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	3,014	3,020
≥ 1.2X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	1,225,302	1,309,699
≥ 1.2X | 0%-59.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	670,461	709,342
≥ 1.2X | 60%-69.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	392,595	442,308
≥ 1.2X | 70%-79.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	160,246	156,049
≥ 1.2X | 80% or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	2,000	2,000
1.0X to 1.2X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	44,765	46,330
1.0X to 1.2X | 0%-59.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	18,919	14,814
1.0X to 1.2X | 60%-69.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	18,656	23,260
1.0X to 1.2X | 70%-79.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	7,190	7,236
1.0X to 1.2X | 80% or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	0	1,020
Less than 1.0X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	1,014	345
Less than 1.0X | 0%-59.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	0	345
Less than 1.0X | 60%-69.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	0	0
Less than 1.0X | 70%-79.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	0	0
Less than 1.0X | 80% or greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	$ 1,014	$ 0