Investments (Credit Quality Indicators) (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses	$ 6,322,544	$ 6,160,410
Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	292,455	1,142,205
One Year Prior	1,140,691	782,796
Two Year Prior	780,341	1,261,597
Three Year Prior	1,262,747	363,599
Four Year Prior	362,213	528,288
Prior	1,901,826	1,514,519
Revolving Loans	0
Recording investment gross of allowance for credit losses	5,740,273	5,593,004
Commercial mortgage loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	252,287	1,038,315
One Year Prior	1,036,927	779,282
Two Year Prior	776,847	1,261,597
Three Year Prior	1,262,747	292,561
Four Year Prior	291,472	497,407
Prior	1,759,044	1,402,831
Revolving Loans	0
Recording investment gross of allowance for credit losses	5,379,324	5,271,993
Commercial mortgage loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	40,168	103,890
One Year Prior	103,764	3,514
Two Year Prior	3,494	0
Three Year Prior	0	0
Four Year Prior	0	15,632
Prior	70,021	40,521
Revolving Loans	0
Recording investment gross of allowance for credit losses	217,447	163,557
Commercial mortgage loans | Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	0	0
One Year Prior	0	0
Two Year Prior	0	0
Three Year Prior	0	71,038
Four Year Prior	70,741	15,249
Prior	72,761	71,167
Revolving Loans	0
Recording investment gross of allowance for credit losses	143,502	157,454
Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	7,035	126,949
One Year Prior	109,655	235,585
Two Year Prior	224,770	132,042
Three Year Prior	131,883	25,875
Four Year Prior	25,686	15,824
Prior	40,265	25,771
Revolving Loans	35,999
Recording investment gross of allowance for credit losses	575,293	562,046
Agricultural Property Loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	6,035	126,949
One Year Prior	109,655	233,585
Two Year Prior	222,770	130,353
Three Year Prior	130,187	24,063
Four Year Prior	23,912	15,824
Prior	40,265	25,771
Revolving Loans	35,999
Recording investment gross of allowance for credit losses	568,823	556,545
Agricultural Property Loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	1,000	0
One Year Prior	0	2,000
Two Year Prior	2,000	0
Three Year Prior	0	1,812
Four Year Prior	1,774	0
Prior	0	0
Revolving Loans	0
Recording investment gross of allowance for credit losses	4,774	3,812
Agricultural Property Loans | Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	0	0
One Year Prior	0	0
Two Year Prior	0	1,689
Three Year Prior	1,696	0
Four Year Prior	0	0
Prior	0	0
Revolving Loans	0
Recording investment gross of allowance for credit losses	1,696	1,689
0%-59.99% | Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	968	249,037
One Year Prior	248,758	245,914
Two Year Prior	245,290	482,718
Three Year Prior	470,775	109,249
Four Year Prior	109,045	265,053
Prior	1,197,318	1,068,763
Revolving Loans	0
Recording investment gross of allowance for credit losses	2,272,154	2,420,734
0%-59.99% | Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	7,035	73,774
One Year Prior	71,353	179,375
Two Year Prior	168,560	132,042
Three Year Prior	131,883	25,875
Four Year Prior	25,686	15,824
Prior	40,265	25,771
Revolving Loans	21,184
Recording investment gross of allowance for credit losses	465,966	452,661
60%-69.99% | Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	206,874	675,153
One Year Prior	674,670	355,984
Two Year Prior	355,359	449,878
Three Year Prior	463,957	172,721
Four Year Prior	171,819	225,803
Prior	421,052	206,237
Revolving Loans	0
Recording investment gross of allowance for credit losses	2,293,731	2,085,776
60%-69.99% | Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	0	47,489
One Year Prior	38,302	56,210
Two Year Prior	56,210	0
Three Year Prior	0	0
Four Year Prior	0	0
Prior	0	0
Revolving Loans	14,815
Recording investment gross of allowance for credit losses	109,327	103,699
70%-79.99% | Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	84,613	218,015
One Year Prior	217,263	133,343
Two Year Prior	133,198	255,299
Three Year Prior	254,023	77,812
Four Year Prior	77,515	20,924
Prior	103,786	86,806
Revolving Loans	0
Recording investment gross of allowance for credit losses	870,398	792,199
70%-79.99% | Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	0	5,686
One Year Prior	0	0
Two Year Prior	0	0
Three Year Prior	0	0
Four Year Prior	0	0
Prior	0	0
Revolving Loans	0
Recording investment gross of allowance for credit losses	0	5,686
80% or greater | Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	0	0
One Year Prior	0	47,555
Two Year Prior	46,494	73,702
Three Year Prior	73,992	3,817
Four Year Prior	3,834	16,508
Prior	179,670	152,713
Revolving Loans	0
Recording investment gross of allowance for credit losses	303,990	294,295
80% or greater | Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current Year	0	0
One Year Prior	0	0
Two Year Prior	0	0
Three Year Prior	0	0
Four Year Prior	0	0
Prior	0	0
Revolving Loans	0
Recording investment gross of allowance for credit losses	$ 0	$ 0