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	$ 1,227,627	$ 1,211,215
0%-59.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	728,685	709,358
60%-69.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	341,268	340,111
70%-79.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	157,447	133,517
80% and greater
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	227	28,229
≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	1,172,884	1,151,820
≥ 1.2X | 0%-59.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	714,155	696,507
≥ 1.2X | 60%-69.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	324,198	321,586
≥ 1.2X | 70%-79.99%
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	134,531	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	53,701	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	13,715	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	17,070	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	22,916	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	0	229
Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	1,042	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	$ 227	$ 0