Credit Quality Indicator for Conventional Mortgage Loans (Details) - USD ($) $ in Thousands		Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Total	[1]	$ 4,531,974	$ 4,501,751
Conventional Mortgage Loan [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Origination Year - Prior to 2016	[2]	1,668,386
Origination Year - 2016 to 2020	[2]	2,575,999
Total	[2]	4,244,385
Total past due	[2]		53,943
Total current loans	[2]		4,171,676
Total mortgage loans	[2]		4,225,619
Past due 30-59 days delinquent [Member] | Conventional Mortgage Loan [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Origination Year - Prior to 2016	[2]	14,166
Origination Year - 2016 to 2020	[2]	14,575
Total	[2]	28,741
Total past due	[2]		36,336
Past due 60-89 days delinquent [Member] | Conventional Mortgage Loan [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Origination Year - Prior to 2016	[2]	3,502
Origination Year - 2016 to 2020	[2]	3,717
Total	[2]	7,219
Total past due	[2]		6,911
Past due 90 days or more delinquent [Member] | Conventional Mortgage Loan [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Origination Year - Prior to 2016	[2]	6,830
Origination Year - 2016 to 2020	[2]	4,129
Total	[2]	10,959
Total past due	[2]		$ 10,696
Nonperforming Financial Instruments [Member] | Conventional Mortgage Loan [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Origination Year - Prior to 2016	[2]	24,498
Origination Year - 2016 to 2020	[2]	22,421
Total	[2]	46,919
Performing Financial Instruments [Member] | Conventional Mortgage Loan [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Origination Year - Prior to 2016	[2]	1,643,888
Origination Year - 2016 to 2020	[2]	2,553,578
Total	[2]	$ 4,197,466

[1] Excludes accrued interest receivable of $23.5 million at both March 31, 2020, and December 31, 2019

[2] The amortized cost at March 31, 2020, excludes accrued interest receivable whereas the recorded investment at December 31, 2019, includes accrued interest receivable.