Equity Securities at Fair Value (Details)	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($) security	Mar. 31, 2019	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Jan. 01, 2019 USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Equity securities	$ 4,706,000			$ 4,706,000				$ 2,855,000
Accounting Standards Update [Extensible List]			us-gaap:AccountingStandardsUpdate201601Member	us-gaap:AccountingStandardsUpdate201613Member		us-gaap:AccountingStandardsUpdate201601Member
Cumulative effect of new accounting principle in period of adoption	1,017,330,000	$ 991,835,000		$ 1,017,330,000	$ 991,835,000	$ 972,060,000	$ 1,040,988,000	982,517,000	$ 1,007,909,000
Change in fair value of equity securities	(4,000)	(59,000)		55,000	189,000
Proceeds from sales of equity securities	0	$ 161,000		0	926,000
Number of equity securities | security		1
Equity securities, realized gain		$ 70,000			196,000
Equity securities, realized loss					0
Retained Earnings
Debt and Equity Securities, FV-NI [Line Items]
Cumulative effect of new accounting principle in period of adoption	652,430,000	601,932,000		652,430,000	601,932,000	560,216,000	637,343,000	615,481,000	587,714,000
Accumulated Other Comprehensive (Loss)
Debt and Equity Securities, FV-NI [Line Items]
Cumulative effect of new accounting principle in period of adoption	$ (87,929,000)	$ (55,972,000)		$ (87,929,000)	$ (55,972,000)	(71,897,000)	$ (53,207,000)	$ (68,735,000)	$ (61,319,000)
Cumulative Effect, Period of Adoption, Adjustment
Debt and Equity Securities, FV-NI [Line Items]
Cumulative effect of new accounting principle in period of adoption						0
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings
Debt and Equity Securities, FV-NI [Line Items]
Cumulative effect of new accounting principle in period of adoption						548,000				$ 548,000
Cumulative Effect, Period of Adoption, Adjustment | Accumulated Other Comprehensive (Loss)
Debt and Equity Securities, FV-NI [Line Items]
Cumulative effect of new accounting principle in period of adoption						$ (548,000)				$ (548,000)