Mortgage Loans on Real Estate (Summary of Mortgage Loan Portfolio) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	[1]	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Valuation allowance	$ (19,279)	$ (20,089)	$ (17,779)		$ (9,179)
Mortgage loans, carrying value	3,926,699				3,448,793
Commercial Mortgage Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	3,778,317				3,458,914
Deferred fees and costs, net	(1,827)				(942)
Amortized cost	3,776,490				3,457,972
Valuation allowance	(17,429)	(18,439)	(17,779)		(9,179)
Mortgage loans, carrying value	3,759,061				3,448,793
Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	171,934				0
Deferred fees and costs, net	392				0
Unamortized discounts and premiums, net	(2,838)				0
Amortized cost	169,488				0
Valuation allowance	(1,850)	$ (1,650)	$ 0		0
Mortgage loans, carrying value	$ 167,638				$ 0

[1] (1) Upon adoption of authoritative guidance effective January 1, 2020, we updated our accounting policies and methodology for calculating the general loan loss allowance, resulting in an adjustment to our mortgage loan valuation allowance. See Note 1 for further details.