Investments (Credit Quality Indicators) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	$ 66,020
2019	174,945
2018	195,538
2017	177,292
2016	263,864
Prior	334,167
Revolving Loans	0
Recording investment gross of allowance for credit losses	1,211,826	$ 1,219,456
Commercial mortgage loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
2020	50,470
2019	164,571
2018	178,985
2017	170,902
2016	233,970
Prior	316,215
Revolving Loans	0
Recording investment gross of allowance for credit losses	1,115,113	1,159,411
Commercial mortgage loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
2020	15,550
2019	10,374
2018	16,553
2017	6,390
2016	29,894
Prior	17,307
Revolving Loans	0
Recording investment gross of allowance for credit losses	96,068	58,948
Commercial mortgage loans | Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	645
Revolving Loans	0
Recording investment gross of allowance for credit losses	645	1,097
Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	6,486
2016	0
Prior	13,449
Revolving Loans	0
Recording investment gross of allowance for credit losses	19,935	22,197
Agricultural Property Loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	6,486
2016	0
Prior	12,634
Revolving Loans	0
Recording investment gross of allowance for credit losses	19,120	21,382
Agricultural Property Loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Recording investment gross of allowance for credit losses	0	0
Agricultural Property Loans | Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	815
Revolving Loans	0
Recording investment gross of allowance for credit losses	815	815
0%-59.99% | Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	10,700
2019	58,862
2018	36,513
2017	82,945
2016	147,385
Prior	254,165
Revolving Loans	0
Recording investment gross of allowance for credit losses	590,570	674,651
0%-59.99% | Commercial mortgage loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		655,136
0%-59.99% | Commercial mortgage loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		18,418
0%-59.99% | Commercial mortgage loans | Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		1,097
0%-59.99% | Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	6,486
2016	0
Prior	13,449
Revolving Loans	0
Recording investment gross of allowance for credit losses	19,935	22,197
0%-59.99% | Agricultural Property Loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		21,382
0%-59.99% | Agricultural Property Loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
0%-59.99% | Agricultural Property Loans | Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		815
60%-69.99% | Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	35,252
2019	68,541
2018	77,956
2017	61,695
2016	106,914
Prior	78,216
Revolving Loans	0
Recording investment gross of allowance for credit losses	428,574	367,626
60%-69.99% | Commercial mortgage loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		354,827
60%-69.99% | Commercial mortgage loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		12,799
60%-69.99% | Commercial mortgage loans | Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
60%-69.99% | Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Recording investment gross of allowance for credit losses	0	0
60%-69.99% | Agricultural Property Loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
60%-69.99% | Agricultural Property Loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
60%-69.99% | Agricultural Property Loans | Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
70%-79.99% | Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	20,068
2019	47,542
2018	81,069
2017	32,652
2016	9,565
Prior	1,786
Revolving Loans	0
Recording investment gross of allowance for credit losses	192,682	176,954
70%-79.99% | Commercial mortgage loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		149,448
70%-79.99% | Commercial mortgage loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		27,506
70%-79.99% | Commercial mortgage loans | Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
70%-79.99% | Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Recording investment gross of allowance for credit losses	0	0
70%-79.99% | Agricultural Property Loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
70%-79.99% | Agricultural Property Loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
70%-79.99% | Agricultural Property Loans | Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
80% or greater | Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Recording investment gross of allowance for credit losses	0	225
80% or greater | Commercial mortgage loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
80% or greater | Commercial mortgage loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		225
80% or greater | Commercial mortgage loans | Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
80% or greater | Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Recording investment gross of allowance for credit losses	$ 0	0
80% or greater | Agricultural Property Loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
80% or greater | Agricultural Property Loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
80% or greater | Agricultural Property Loans | Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		$ 0