Equity Securities at Fair Value (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2018	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Jan. 01, 2019
Debt and Equity Securities, FV-NI [Line Items]
Equity securities	$ 4,710,000		$ 4,710,000				$ 2,855,000
Cumulative effect of new accounting principle in period of adoption	1,040,988,000	$ 1,007,909,000	1,040,988,000	$ 1,007,909,000	$ 972,060,000	$ 961,126,000	982,517,000	$ 994,502,000
Change in fair value of equity securities	643,000	71,000	59,000	247,000
Proceeds from sales of equity securities	0	0	0	765,000
Equity securities, realized gain				126,000
Equity securities, realized loss				0
Accounting Standards Update [Extensible List]					us-gaap:AccountingStandardsUpdate201601Member
Retained Earnings
Debt and Equity Securities, FV-NI [Line Items]
Cumulative effect of new accounting principle in period of adoption	637,343,000	587,714,000	637,343,000	587,714,000	$ 560,216,000	622,246,000	615,481,000	575,683,000
Accumulated Other Comprehensive (Loss)
Debt and Equity Securities, FV-NI [Line Items]
Cumulative effect of new accounting principle in period of adoption	$ (53,207,000)	$ (61,319,000)	$ (53,207,000)	$ (61,319,000)	(71,897,000)	$ (59,665,000)	$ (68,735,000)	$ (65,820,000)
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)