Mortgage Loans on Real Estate (Summary of Mortgage Loan Portfolio) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Valuation allowance	$ (26,581)	$ (28,514)	$ (31,029)	$ (20,089)	$ (19,776)	$ (17,779)
Mortgage loans, carrying value	4,299,945		4,165,489
Commercial Mortgage Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	3,459,879		3,580,154
Deferred fees and costs, net	(1,409)		(1,266)
Mortgage loans, amortized cost	3,458,470		3,578,888
Valuation allowance	(22,498)	(26,139)	(25,529)	(18,119)	(19,604)	(17,579)
Mortgage loans, carrying value	3,435,972		3,553,359
Agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	277,427		245,807
Deferred fees and costs, net	(708)		(634)
Mortgage loans, amortized cost	276,719		245,173
Valuation allowance	(454)	(439)	(2,130)	(320)	(172)	(200)
Mortgage loans, carrying value	276,265		243,043
Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	573,638		366,320
Deferred fees and costs, net	1,487		925
Unamortized discounts and premiums, net	16,212		5,212
Mortgage loans, amortized cost	591,337		372,457
Valuation allowance	(3,629)	$ (1,936)	(3,370)	$ (1,650)	$ 0	$ 0
Mortgage loans, carrying value	$ 587,708		$ 369,087