Mortgage Loans on Real Estate (Mortgage Loan Portfolio Summarized by Geographic Region and Property Type) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Commercial Mortgage and Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrued interest receivable	$ 14,900
Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	171,934	$ 0
Commercial Mortgage Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 3,778,317	$ 3,458,914
Percent	100.00%	100.00%
Commercial Mortgage Loans | East
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 701,945	$ 645,991
Percent	18.60%	18.70%
Commercial Mortgage Loans | Middle Atlantic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 284,211	$ 284,597
Percent	7.50%	8.20%
Commercial Mortgage Loans | Mountain
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 447,623	$ 389,892
Percent	11.80%	11.30%
Commercial Mortgage Loans | New England
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 24,921	$ 9,152
Percent	0.60%	0.30%
Commercial Mortgage Loans | Pacific
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 788,267	$ 655,518
Percent	20.90%	19.00%
Commercial Mortgage Loans | South Atlantic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 781,906	$ 751,199
Percent	20.70%	21.70%
Commercial Mortgage Loans | West North Central
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 297,526	$ 302,534
Percent	7.90%	8.70%
Commercial Mortgage Loans | West South Central
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 451,918	$ 420,031
Percent	12.00%	12.10%
Commercial Mortgage Loans | Office
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 250,523	$ 250,287
Percent	6.60%	7.30%
Commercial Mortgage Loans | Medical Office
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 21,022	$ 29,990
Percent	0.60%	0.90%
Commercial Mortgage Loans | Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 1,199,877	$ 1,225,670
Percent	31.80%	35.40%
Commercial Mortgage Loans | Industrial/Warehouse
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 959,819	$ 896,558
Percent	25.40%	25.90%
Commercial Mortgage Loans | Apartment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 941,858	$ 858,679
Percent	24.90%	24.80%
Commercial Mortgage Loans | Agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 208,303	$ 51,303
Percent	5.50%	1.50%
Commercial Mortgage Loans | Mixed Use/Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 196,915	$ 146,427
Percent	5.20%	4.20%