Mortgage Loans on Real Estate (Commercial Mortgage Loan Portfolio Summarized by Geographic Region and Property Type) (Details) - Commercial Mortgage Loans - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 3,570,314	$ 3,560,903
Percent	100.00%	100.00%
East
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 491,711	$ 502,659
Percent	13.80%	14.10%
Middle Atlantic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 279,732	$ 280,993
Percent	7.80%	7.90%
Mountain
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 413,867	$ 416,307
Percent	11.60%	11.70%
New England
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 75,432	$ 73,631
Percent	2.10%	2.10%
Pacific
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 854,742	$ 858,812
Percent	23.90%	24.10%
South Atlantic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 949,734	$ 934,007
Percent	26.60%	26.20%
West North Central
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 199,149	$ 205,568
Percent	5.60%	5.80%
West South Central
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 305,947	$ 288,926
Percent	8.60%	8.10%
Office
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 369,223	$ 388,978
Percent	10.30%	10.90%
Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 858,628	$ 896,351
Percent	24.00%	25.20%
Industrial/Warehouse
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 909,077	$ 866,623
Percent	25.50%	24.30%
Apartment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 1,029,694	$ 912,984
Percent	28.80%	25.60%
Hotel
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 324,271	$ 285,271
Percent	9.10%	8.00%
Mixed Use/Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 79,421	$ 210,696
Percent	2.30%	6.00%