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	$ 1,231,964	$ 1,211,215
0%-59.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	692,555	709,358
60%-69.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	358,897	340,111
70%-79.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	180,286	133,517
80% and greater
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	226	28,229
≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	1,171,777	1,151,820
≥ 1.2X | 0%-59.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	671,121	696,507
≥ 1.2X | 60%-69.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	348,053	321,586
≥ 1.2X | 70%-79.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	152,603	105,727
≥ 1.2X | 80% and greater
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	0	28,000
1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	59,372	59,315
1.0X to 1.2X | 0%-59.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	20,619	12,771
1.0X to 1.2X | 60%-69.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	10,844	18,525
1.0X to 1.2X | 70%-79.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	27,683	27,790
1.0X to 1.2X | 80% and greater
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	226	229
Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	815	80
Less than 1.0X | 0%-59.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	815	80
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% and greater
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	$ 0	$ 0