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	$ 1,214,676	$ 1,211,215
0%-59.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	712,578	709,358
60%-69.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	333,001	340,111
70%-79.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	141,869	133,517
80% and greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	27,228	28,229
≥ 1.2X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	1,155,899	1,151,820
≥ 1.2X | 0%-59.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	698,110	696,507
≥ 1.2X | 60%-69.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	316,532	321,586
≥ 1.2X | 70%-79.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	114,257	105,727
≥ 1.2X | 80% and greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	27,000	28,000
1.0X to 1.2X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	58,549	59,315
1.0X to 1.2X | 0%-59.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	14,468	12,771
1.0X to 1.2X | 60%-69.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	16,469	18,525
1.0X to 1.2X | 70%-79.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	27,612	27,790
1.0X to 1.2X | 80% and greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	0	229
Less than 1.0X
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	228	80
Less than 1.0X | 0%-59.99%
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	0	80
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% and greater
Financing Receivable, Recorded Investment [Line Items]
Recording investment gross of allowance for credit losses	$ 228	$ 0