Mortgage Loans on Real Estate (Summary By Debt Service Coverage and Loan to Value Ratios) (Details) - Commercial Mortgage Loans - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
2020 - Amortized cost	$ 518,719
2020 - Average LTV	64.00%
2019 - Amortized cost	$ 799,514
2019 - Average LTV	66.00%
2018 - Amortized cost	$ 574,537
2018 - Average LTV	63.00%
2017 - Amortized cost	$ 480,158
2017 - Average LTV	60.00%
2016 - Amortized cost	$ 432,635
2016 - Average LTV	56.00%
Prior - Amortized cost	$ 1,021,551
Prior - Average LTV	48.00%
Amortized cost	$ 3,827,114	$ 3,457,972
Total - Average LTV	58.00%
Debt Service Coverage Ratio: Greater Than or Equal to 1.5
Financing Receivable, Credit Quality Indicator [Line Items]
2020 - Amortized cost	$ 316,727
2020 - Average LTV	64.00%
2019 - Amortized cost	$ 455,910
2019 - Average LTV	67.00%
2018 - Amortized cost	$ 405,363
2018 - Average LTV	61.00%
2017 - Amortized cost	$ 310,373
2017 - Average LTV	58.00%
2016 - Amortized cost	$ 378,311
2016 - Average LTV	55.00%
Prior - Amortized cost	$ 816,947
Prior - Average LTV	46.00%
Amortized cost	$ 2,683,631
Total - Average LTV	56.00%
Debt Service Coverage Ratio: Greater Than or Equal to 1.2 and Less Than 1.5
Financing Receivable, Credit Quality Indicator [Line Items]
2020 - Amortized cost	$ 187,242
2020 - Average LTV	63.00%
2019 - Amortized cost	$ 256,913
2019 - Average LTV	69.00%
2018 - Amortized cost	$ 141,693
2018 - Average LTV	68.00%
2017 - Amortized cost	$ 151,241
2017 - Average LTV	65.00%
2016 - Amortized cost	$ 54,324
2016 - Average LTV	62.00%
Prior - Amortized cost	$ 159,528
Prior - Average LTV	54.00%
Amortized cost	$ 950,941
Total - Average LTV	64.00%
Debt Service Coverage Ratio: Greater Than or Equal to 1.0 and Less Than 1.2
Financing Receivable, Credit Quality Indicator [Line Items]
2020 - Amortized cost	$ 5,966
2020 - Average LTV	68.00%
2019 - Amortized cost	$ 61,471
2019 - Average LTV	58.00%
2018 - Amortized cost	$ 8,931
2018 - Average LTV	75.00%
2017 - Amortized cost	$ 15,548
2017 - Average LTV	64.00%
2016 - Amortized cost	$ 0
2016 - Average LTV	0.00%
Prior - Amortized cost	$ 22,868
Prior - Average LTV	63.00%
Amortized cost	$ 114,784
Total - Average LTV	62.00%
Debt Service Coverage Ratio: Less Than 1.0
Financing Receivable, Credit Quality Indicator [Line Items]
2020 - Amortized cost	$ 8,784
2020 - Average LTV	60.00%
2019 - Amortized cost	$ 25,220
2019 - Average LTV	44.00%
2018 - Amortized cost	$ 18,550
2018 - Average LTV	65.00%
2017 - Amortized cost	$ 2,996
2017 - Average LTV	62.00%
2016 - Amortized cost	$ 0
2016 - Average LTV	0.00%
Prior - Amortized cost	$ 22,208
Prior - Average LTV	51.00%
Amortized cost	$ 77,758
Total - Average LTV	54.00%