Mortgage Loans on Real Estate (Summary By Debt Service Coverage and Loan to Value Ratios) (Details) - Commercial Real Estate Portfolio Segment - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
2020 - Amortized cost	$ 321,403
2020 - Average LTV	65.00%
2019 - Amortized cost	$ 800,009
2019 - Average LTV	68.00%
2018 - Amortized cost	$ 576,980
2018 - Average LTV	64.00%
2017 - Amortized cost	$ 483,594
2017 - Average LTV	61.00%
2016 - Amortized cost	$ 436,381
2016 - Average LTV	56.00%
Prior - Amortized cost	$ 1,070,034
Prior - Average LTV	48.00%
Amortized cost	$ 3,688,401	$ 3,457,972
Total - Average LTV	59.00%
Debt Service Coverage Ratio, Greater Than or Equal to 1.5
Financing Receivable, Credit Quality Indicator [Line Items]
2020 - Amortized cost	$ 209,636
2020 - Average LTV	63.00%
2019 - Amortized cost	$ 463,287
2019 - Average LTV	67.00%
2018 - Amortized cost	$ 411,362
2018 - Average LTV	61.00%
2017 - Amortized cost	$ 308,703
2017 - Average LTV	58.00%
2016 - Amortized cost	$ 394,953
2016 - Average LTV	56.00%
Prior - Amortized cost	$ 863,496
Prior - Average LTV	47.00%
Amortized cost	$ 2,651,437
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	$ 108,167
2020 - Average LTV	68.00%
2019 - Amortized cost	$ 299,053
2019 - Average LTV	69.00%
2018 - Amortized cost	$ 145,090
2018 - Average LTV	71.00%
2017 - Amortized cost	$ 159,166
2017 - Average LTV	66.00%
2016 - Amortized cost	$ 37,411
2016 - Average LTV	60.00%
Prior - Amortized cost	$ 138,857
Prior - Average LTV	54.00%
Amortized cost	$ 887,744
Total - Average LTV	66.00%
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	$ 3,600
2020 - Average LTV	68.00%
2019 - Amortized cost	$ 36,219
2019 - Average LTV	61.00%
2018 - Amortized cost	$ 11,056
2018 - Average LTV	73.00%
2017 - Amortized cost	$ 15,725
2017 - Average LTV	65.00%
2016 - Amortized cost	$ 4,017
2016 - Average LTV	47.00%
Prior - Amortized cost	$ 45,125
Prior - Average LTV	60.00%
Amortized cost	$ 115,742
Total - Average LTV	62.00%
Debt Service Coverage Ratio, Less Than 1.0
Financing Receivable, Credit Quality Indicator [Line Items]
2020 - Amortized cost	$ 0
2020 - Average LTV	0.00%
2019 - Amortized cost	$ 1,450
2019 - Average LTV	43.00%
2018 - Amortized cost	$ 9,472
2018 - Average LTV	70.00%
2017 - Amortized cost	$ 0
2017 - Average LTV	0.00%
2016 - Amortized cost	$ 0
2016 - Average LTV	0.00%
Prior - Amortized cost	$ 22,556
Prior - Average LTV	52.00%
Amortized cost	$ 33,478
Total - Average LTV	57.00%