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	$ 40,707
2019	342,232
2018	105,962
2017	280,342
2016	283,225
Prior	370,692
Revolving Loans	0
Recording investment gross of allowance for credit losses	1,423,160
Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	22,500
2019	13,798
2018	1,261
2017	8,553
2016	1,180
Prior	34,741
Revolving Loans	0
Recording investment gross of allowance for credit losses	82,033
Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	63,207
2019	356,030
2018	107,223
2017	288,895
2016	284,405
Prior	405,433
Revolving Loans	0
Recording investment gross of allowance for credit losses	1,505,193	$ 1,474,185
0%-59.99% | Commercial Mortgage Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	112,817
2018	25,889
2017	95,941
2016	160,898
Prior	257,347
Revolving Loans	0
Recording investment gross of allowance for credit losses	652,892
0%-59.99% | Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	22,500
2019	13,798
2018	1,261
2017	8,553
2016	1,180
Prior	34,741
Revolving Loans	0
Recording investment gross of allowance for credit losses	82,033
0%-59.99% | Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	22,500
2019	126,615
2018	27,150
2017	104,494
2016	162,078
Prior	292,088
Revolving Loans	0
Recording investment gross of allowance for credit losses	734,925	725,904
60%-69.99% | Commercial Mortgage Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	33,614
2019	163,428
2018	25,274
2017	108,435
2016	93,846
Prior	107,170
Revolving Loans	0
Recording investment gross of allowance for credit losses	531,767
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	33,614
2019	163,428
2018	25,274
2017	108,435
2016	93,846
Prior	107,170
Revolving Loans	0
Recording investment gross of allowance for credit losses	531,767	513,571
70%-79.99% | Commercial Mortgage Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	7,093
2019	65,987
2018	54,799
2017	67,721
2016	28,481
Prior	5,184
Revolving Loans	0
Recording investment gross of allowance for credit losses	229,265
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	7,093
2019	65,987
2018	54,799
2017	67,721
2016	28,481
Prior	5,184
Revolving Loans	0
Recording investment gross of allowance for credit losses	229,265	233,713
80% or greater | Commercial Mortgage Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	8,245
2016	0
Prior	991
Revolving Loans	0
Recording investment gross of allowance for credit losses	9,236
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	8,245
2016	0
Prior	991
Revolving Loans	0
Recording investment gross of allowance for credit losses	$ 9,236	997
≥ 1.2X | Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		1,418,305
≥ 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		715,654
≥ 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		499,493
≥ 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		203,158
≥ 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		52,271
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		6,641
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		14,078
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		30,555
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		997
Less than 1.0X | Commercial Mortgage and Agricultural Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		3,609
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		3,609
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