Mortgage Loans on Real Estate (Mortgage Loan Portfolio Summarized by Geographic Region and Property Type) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Commercial Mortgage and Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrued interest receivable	$ 15,500
Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	153,913	$ 0
Commercial Mortgage Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 3,828,972	$ 3,458,914
Percent	100.00%	100.00%
Commercial Mortgage Loans | East
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 714,198	$ 645,991
Percent	18.70%	18.70%
Commercial Mortgage Loans | Middle Atlantic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 287,864	$ 284,597
Percent	7.50%	8.20%
Commercial Mortgage Loans | Mountain
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 444,495	$ 389,892
Percent	11.60%	11.30%
Commercial Mortgage Loans | New England
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 25,066	$ 9,152
Percent	0.70%	0.30%
Commercial Mortgage Loans | Pacific
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 782,240	$ 655,518
Percent	20.40%	19.00%
Commercial Mortgage Loans | South Atlantic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 820,260	$ 751,199
Percent	21.40%	21.70%
Commercial Mortgage Loans | West North Central
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 308,874	$ 302,534
Percent	8.10%	8.70%
Commercial Mortgage Loans | West South Central
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 445,975	$ 420,031
Percent	11.60%	12.10%
Commercial Mortgage Loans | Office
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 259,888	$ 250,287
Percent	6.80%	7.30%
Commercial Mortgage Loans | Medical Office
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 27,372	$ 29,990
Percent	0.70%	0.90%
Commercial Mortgage Loans | Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 1,225,212	$ 1,225,670
Percent	32.00%	35.40%
Commercial Mortgage Loans | Industrial/Warehouse
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 974,579	$ 896,558
Percent	25.50%	25.90%
Commercial Mortgage Loans | Apartment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 971,536	$ 858,679
Percent	25.40%	24.80%
Commercial Mortgage Loans | Agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 177,333	$ 51,303
Percent	4.60%	1.50%
Commercial Mortgage Loans | Mixed Use/Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 193,052	$ 146,427
Percent	5.00%	4.20%