Mortgage Loans on Real Estate (Summary By Debt Service Coverage and Loan to Value Ratios) (Details) - Commercial Mortgage Loans - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
2020 - Amortized cost	$ 557,905
2020 - Average LTV	62.00%
2019 - Amortized cost	$ 798,373
2019 - Average LTV	66.00%
2018 - Amortized cost	$ 557,492
2018 - Average LTV	62.00%
2017 - Amortized cost	$ 475,303
2017 - Average LTV	60.00%
2016 - Amortized cost	$ 428,018
2016 - Average LTV	55.00%
Prior - Amortized cost	$ 959,399
Prior - Average LTV	47.00%
Amortized cost	$ 3,776,490	$ 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	$ 337,455
2020 - Average LTV	62.00%
2019 - Amortized cost	$ 461,926
2019 - Average LTV	67.00%
2018 - Amortized cost	$ 422,523
2018 - Average LTV	61.00%
2017 - Amortized cost	$ 319,071
2017 - Average LTV	57.00%
2016 - Amortized cost	$ 374,917
2016 - Average LTV	54.00%
Prior - Amortized cost	$ 787,122
Prior - Average LTV	46.00%
Amortized cost	$ 2,703,014
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	$ 204,841
2020 - Average LTV	60.00%
2019 - Amortized cost	$ 240,432
2019 - Average LTV	68.00%
2018 - Amortized cost	$ 119,076
2018 - Average LTV	67.00%
2017 - Amortized cost	$ 138,287
2017 - Average LTV	66.00%
2016 - Amortized cost	$ 53,101
2016 - Average LTV	64.00%
Prior - Amortized cost	$ 122,339
Prior - Average LTV	53.00%
Amortized cost	$ 878,076
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	$ 6,824
2020 - Average LTV	66.00%
2019 - Amortized cost	$ 54,341
2019 - Average LTV	57.00%
2018 - Amortized cost	$ 2,793
2018 - Average LTV	72.00%
2017 - Amortized cost	$ 7,698
2017 - Average LTV	65.00%
2016 - Amortized cost	$ 0
2016 - Average LTV	0.00%
Prior - Amortized cost	$ 28,700
Prior - Average LTV	64.00%
Amortized cost	$ 100,356
Total - Average LTV	61.00%
Debt Service Coverage Ratio: Less Than 1.0
Financing Receivable, Credit Quality Indicator [Line Items]
2020 - Amortized cost	$ 8,785
2020 - Average LTV	60.00%
2019 - Amortized cost	$ 41,674
2019 - Average LTV	56.00%
2018 - Amortized cost	$ 13,100
2018 - Average LTV	65.00%
2017 - Amortized cost	$ 10,247
2017 - Average LTV	69.00%
2016 - Amortized cost	$ 0
2016 - Average LTV	0.00%
Prior - Amortized cost	$ 21,238
Prior - Average LTV	56.00%
Amortized cost	$ 95,044
Total - Average LTV	59.00%