Investments (Loan-to-value ratios) (Details) $ in Thousands	Jun. 30, 2020 USD ($)
Commercial Mortgage Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	$ 173,149
2019	348,988
2018	106,494
2017	280,983
2016	284,788
Prior	360,375
Revolving Loans	0
Recording investment gross of allowance for credit losses	1,554,777
Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	34,465
2019	13,798
2018	1,261
2017	8,553
2016	1,180
Prior	33,257
Revolving Loans	0
Recording investment gross of allowance for credit losses	92,514
Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	207,614
2019	362,786
2018	107,755
2017	289,536
2016	285,968
Prior	393,632
Revolving Loans	0
Recording investment gross of allowance for credit losses	1,647,291
0%-59.99% | Commercial Mortgage Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	115,190
2018	26,510
2017	82,913
2016	162,983
Prior	247,331
Revolving Loans	0
Recording investment gross of allowance for credit losses	634,927
0%-59.99% | Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	34,465
2019	13,798
2018	1,261
2017	8,553
2016	1,180
Prior	33,257
Revolving Loans	0
Recording investment gross of allowance for credit losses	92,514
0%-59.99% | Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	34,465
2019	128,988
2018	27,771
2017	91,466
2016	164,163
Prior	280,588
Revolving Loans	0
Recording investment gross of allowance for credit losses	727,441
60%-69.99% | Commercial Mortgage Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	125,732
2019	165,219
2018	32,186
2017	121,512
2016	93,427
Prior	104,153
Revolving Loans	0
Recording investment gross of allowance for credit losses	642,229
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	125,732
2019	165,219
2018	32,186
2017	121,512
2016	93,427
Prior	104,153
Revolving Loans	0
Recording investment gross of allowance for credit losses	642,229
70%-79.99% | Commercial Mortgage Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	47,417
2019	68,579
2018	47,798
2017	76,558
2016	28,378
Prior	7,906
Revolving Loans	0
Recording investment gross of allowance for credit losses	276,636
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	47,417
2019	68,579
2018	47,798
2017	76,558
2016	28,378
Prior	7,906
Revolving Loans	0
Recording investment gross of allowance for credit losses	276,636
80% or greater | Commercial Mortgage Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	985
Revolving Loans	0
Recording investment gross of allowance for credit losses	985
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	985
Revolving Loans	0
Recording investment gross of allowance for credit losses	$ 985