Investments (Credit Quality Indicators) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	$ 2,049,077	$ 1,773,152
Commercial Mortgage Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	824,988	256,178
One Years Prior	119,659	370,081
Two Years Prior	322,417	107,164
Three Years Prior	87,360	287,341
Four Years Prior	167,976	289,478
Prior	434,710	358,336
Recording investment gross of allowance for credit losses	1,957,110	1,668,578
Commercial Mortgage Loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	802,988	256,178
One Years Prior	119,659	347,151
Two Years Prior	291,455	107,164
Three Years Prior	76,739	287,341
Four Years Prior	134,416	274,124
Prior	409,620	323,060
Recording investment gross of allowance for credit losses	1,834,877	1,595,018
Commercial Mortgage Loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	22,000	0
One Years Prior	0	22,930
Two Years Prior	19,890	0
Three Years Prior	10,621	0
Four Years Prior	12,278	3,969
Prior	4,846	18,420
Recording investment gross of allowance for credit losses	69,635	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	11,072	0
Three Years Prior	0	0
Four Years Prior	21,282	11,385
Prior	20,244	16,856
Recording investment gross of allowance for credit losses	52,598	28,241
Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	38,000	49,507
One Years Prior	26,970	13,769
Two Years Prior	13,711	1,255
Three Years Prior	1,238	7,493
Four Years Prior	2,701	1,180
Prior	9,347	31,370
Recording investment gross of allowance for credit losses	91,967	104,574
Agricultural Property Loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	38,000	49,507
One Years Prior	26,970	13,769
Two Years Prior	13,711	1,255
Three Years Prior	1,238	4,277
Four Years Prior	2,701	1,180
Prior	5,908	27,783
Recording investment gross of allowance for credit losses	88,528	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	3,439	3,587
Recording investment gross of allowance for credit losses	3,439	6,803
0%-59.99% | Commercial Mortgage Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	157,985	0
One Years Prior	0	114,636
Two Years Prior	103,674	36,423
Three Years Prior	29,382	116,130
Four Years Prior	96,901	175,740
Prior	291,073	255,848
Recording investment gross of allowance for credit losses	679,015	698,777
0%-59.99% | Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	38,000	47,245
One Years Prior	26,970	13,769
Two Years Prior	13,711	1,255
Three Years Prior	1,238	7,493
Four Years Prior	2,701	1,180
Prior	9,347	31,370
Recording investment gross of allowance for credit losses	91,967	102,312
60%-69.99% | Commercial Mortgage Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	393,844	174,507
One Years Prior	47,418	204,112
Two Years Prior	155,279	59,935
Three Years Prior	57,978	90,954
Four Years Prior	49,793	57,569
Prior	49,700	54,530
Recording investment gross of allowance for credit losses	754,012	641,607
60%-69.99% | Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	0	2,262
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	2,262
70%-79.99% | Commercial Mortgage Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	273,159	81,671
One Years Prior	72,241	51,333
Two Years Prior	63,464	10,806
Three Years Prior	0	80,257
Four Years Prior	21,282	56,169
Prior	73,576	46,855
Recording investment gross of allowance for credit losses	503,722	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	20,361	1,103
Recording investment gross of allowance for credit losses	20,361	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