Investments (Credit Quality Indicators) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Commercial Mortgage Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	$ 52,635	$ 256,178
One Years Prior	255,650	370,081
Two Years Prior	365,603	107,164
Three Years Prior	106,655	287,341
Four Years Prior	285,341	289,478
Prior	639,652	358,336
Recording investment gross of allowance for credit losses	1,705,536	1,668,578
Commercial Mortgage Loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	52,635	256,178
One Years Prior	255,650	347,151
Two Years Prior	342,820	107,164
Three Years Prior	106,655	287,341
Four Years Prior	285,341	274,124
Prior	589,367	323,060
Recording investment gross of allowance for credit losses	1,632,468	1,595,018
Commercial Mortgage Loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	0	0
One Years Prior	0	22,930
Two Years Prior	22,783	0
Three Years Prior	0	0
Four Years Prior	0	3,969
Prior	22,084	18,420
Recording investment gross of allowance for credit losses	44,867	45,319
Commercial Mortgage Loans | Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	0	0
One Years Prior	0	0
Two Years Prior	0	0
Three Years Prior	0	0
Four Years Prior	0	11,385
Prior	28,201	16,856
Recording investment gross of allowance for credit losses	28,201	28,241
Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	3,750	49,507
One Years Prior	49,311	13,769
Two Years Prior	13,740	1,255
Three Years Prior	1,249	7,493
Four Years Prior	0	1,180
Prior	28,213	31,370
Recording investment gross of allowance for credit losses	96,263	104,574
Agricultural Property Loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	3,750	49,507
One Years Prior	49,311	13,769
Two Years Prior	13,740	1,255
Three Years Prior	1,249	4,277
Four Years Prior	0	1,180
Prior	24,632	27,783
Recording investment gross of allowance for credit losses	92,682	97,771
Agricultural Property Loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	0	0
One Years Prior	0	0
Two Years Prior	0	0
Three Years Prior	0	0
Four Years Prior	0	0
Prior	0	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]
Current Year	0	0
One Years Prior	0	0
Two Years Prior	0	0
Three Years Prior	0	3,216
Four Years Prior	0	0
Prior	3,581	3,587
Recording investment gross of allowance for credit losses	3,581	6,803
0%-59.99% | Commercial Mortgage Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	38,560	0
One Years Prior	0	114,636
Two Years Prior	113,410	36,423
Three Years Prior	43,037	116,130
Four Years Prior	115,447	175,740
Prior	427,874	255,848
Recording investment gross of allowance for credit losses	738,328	698,777
0%-59.99% | Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	3,750	47,245
One Years Prior	47,126	13,769
Two Years Prior	13,740	1,255
Three Years Prior	1,249	7,493
Four Years Prior	0	1,180
Prior	28,213	31,370
Recording investment gross of allowance for credit losses	94,078	102,312
60%-69.99% | Commercial Mortgage Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	14,075	174,507
One Years Prior	175,627	204,112
Two Years Prior	201,021	59,935
Three Years Prior	52,873	90,954
Four Years Prior	116,336	57,569
Prior	103,477	54,530
Recording investment gross of allowance for credit losses	663,409	641,607
60%-69.99% | Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	0	2,262
One Years Prior	2,185	0
Two Years Prior	0	0
Three Years Prior	0	0
Four Years Prior	0	0
Prior	0	0
Recording investment gross of allowance for credit losses	2,185	2,262
70%-79.99% | Commercial Mortgage Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	0	81,671
One Years Prior	80,023	51,333
Two Years Prior	51,172	10,806
Three Years Prior	10,745	80,257
Four Years Prior	53,558	56,169
Prior	107,204	46,855
Recording investment gross of allowance for credit losses	302,702	327,091
70%-79.99% | Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	0	0
One Years Prior	0	0
Two Years Prior	0	0
Three Years Prior	0	0
Four Years Prior	0	0
Prior	0	0
Recording investment gross of allowance for credit losses	0	0
80% or greater | Commercial Mortgage Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	0	0
One Years Prior	0	0
Two Years Prior	0	0
Three Years Prior	0	0
Four Years Prior	0	0
Prior	1,097	1,103
Recording investment gross of allowance for credit losses	1,097	1,103
80% or greater | Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	0	0
One Years Prior	0	0
Two Years Prior	0	0
Three Years Prior	0	0
Four Years Prior	0	0
Prior	0	0
Recording investment gross of allowance for credit losses	$ 0	$ 0