General (Details)	9 Months Ended
	Sep. 30, 2020 USD ($) segment	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)
Number of operating segments | segment	1
Provision for federal or state income taxes	$ 0
Cumulative effect of the adoption of the ASC 842		$ 778,821,000	$ 787,305,000	$ 806,055,000	$ 785,426,000	$ 793,894,000	$ 787,610,000	$ 788,153,000		$ 832,971,000
Interest receivable		31,248,000			26,586,000
Mortgage Loans Receivable
Expected credit losses for financial instruments
Balance at the Beginning of the Period	2,560,000
Increase/(Decrease) in Expected Credit Loss	2,596,000	2,596,000			2,560,000
Balance at the End of the Period	2,596,000
Notes Receivable
Expected credit losses for financial instruments
Balance at the Beginning of the Period	181,000
Increase/(Decrease) in Expected Credit Loss	144,000	$ 144,000			181,000
Balance at the End of the Period	144,000
Cumulative Effect of Adoption
Cumulative effect of the adoption of the ASC 842										$ (42,808,000)
ASU 2016-02 | Cumulative Effect of Adoption
Cumulative effect of the adoption of the ASC 842									$ 42,808,000
Restatement Adjustment | ASU 2016-13 | Mortgage Loans Receivable
Expected credit losses for financial instruments
Balance at the Beginning of the Period	36,000
Increase/(Decrease) in Expected Credit Loss	36,000				36,000
Restatement Adjustment | ASU 2016-13 | Notes Receivable
Expected credit losses for financial instruments
Balance at the Beginning of the Period	(37,000)
Increase/(Decrease) in Expected Credit Loss	$ (37,000)				$ (37,000)