Investments (Loan-to-value ratios) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	$ 20,790
2019	165,846
2018	193,554
2017	165,789
2016	262,026
Prior	380,362
Revolving Loans	0
Recording investment gross of allowance for credit losses	1,188,367
Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	6,486
2016	0
Prior	15,435
Revolving Loans	0
Recording investment gross of allowance for credit losses	21,921
Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	20,790
2019	165,846
2018	193,554
2017	172,275
2016	262,026
Prior	395,797
Revolving Loans	0
Recording investment gross of allowance for credit losses	1,210,288	$ 1,241,653
0%-59.99% | Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	56,749
2018	34,785
2017	90,588
2016	167,373
Prior	283,170
Revolving Loans	0
Recording investment gross of allowance for credit losses	632,665
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	15,435
Revolving Loans	0
Recording investment gross of allowance for credit losses	21,921
0%-59.99% | Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	56,749
2018	34,785
2017	97,074
2016	167,373
Prior	298,605
Revolving Loans	0
Recording investment gross of allowance for credit losses	654,586	696,847
60%-69.99% | Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	20,013
2019	64,589
2018	70,332
2017	42,202
2016	84,963
Prior	96,041
Revolving Loans	0
Recording investment gross of allowance for credit losses	378,140
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
60%-69.99% | Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	20,013
2019	64,589
2018	70,332
2017	42,202
2016	84,963
Prior	96,041
Revolving Loans	0
Recording investment gross of allowance for credit losses	378,140	367,627
70%-79.99% | Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	777
2019	44,508
2018	88,437
2017	32,999
2016	9,690
Prior	927
Revolving Loans	0
Recording investment gross of allowance for credit losses	177,338
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
70%-79.99% | Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	777
2019	44,508
2018	88,437
2017	32,999
2016	9,690
Prior	927
Revolving Loans	0
Recording investment gross of allowance for credit losses	177,338	176,954
80% or greater | Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	224
Revolving Loans	0
Recording investment gross of allowance for credit losses	224
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
80% or greater | Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	224
Revolving Loans	0
Recording investment gross of allowance for credit losses	$ 224	225
≥ 1.2X | Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		1,180,793
≥ 1.2X | 0%-59.99% | Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		676,517
≥ 1.2X | 60%-69.99% | Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		354,828
≥ 1.2X | 70%-79.99% | Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		149,448
≥ 1.2X | 80% or greater | Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
1.0X to 1.2X | Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		58,948
1.0X to 1.2X | 0%-59.99% | Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		18,418
1.0X to 1.2X | 60%-69.99% | Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		12,799
1.0X to 1.2X | 70%-79.99% | Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		27,506
1.0X to 1.2X | 80% or greater | Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		225
Less than 1.0X | Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		1,912
Less than 1.0X | 0%-59.99% | Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		1,912
Less than 1.0X | 60%-69.99% | Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
Less than 1.0X | 70%-79.99% | Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
Less than 1.0X | 80% or greater | Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		$ 0