Fair Value Measurements (Summarize Changes in Level 3 Assets) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, impairments		$ 0
Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	$ 15,558	29,948
Assets, earnings		(6,794)
Assets, other comprehensive income		61
Assets, purchases		17,369
Assets, (sales)		(39,915)
Assets. transfers out		14,889
Assets, ending balance		15,558
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities, beginning balance	0	793
Liabilities, earnings		133
Liabilities, other comprehensive income		0
Liabilities, impairments		0
Liabilities, purchases		7,609
Liabilities, (sales)		(8,535)
Liabilities, maturities		0
Liabilities, transfers out		0
Liabilities, ending balance		0
Level 3 | Debt securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	0	99
Assets, earnings		(26)
Assets, other comprehensive income		61
Assets, impairments		0
Assets, purchases		0
Assets, (sales)		(134)
Assets. transfers out		0
Assets, ending balance		0
Level 3 | Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	0	99
Assets, earnings		(26)
Assets, other comprehensive income		61
Assets, impairments		0
Assets, purchases		0
Assets, (sales)		(134)
Assets, maturities		0
Assets. transfers out		0
Assets, ending balance		0
Level 3 | Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	15,558	12,541
Assets, earnings	186	1,963
Assets, other comprehensive income	0	0
Assets, impairments	0	0
Assets, purchases	3,000	2,602
Assets, (sales)	(1,066)	(1,548)
Assets, maturities		0
Assets. transfers out	0	0
Assets, ending balance	17,678	15,558
Level 3 | Common stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	9,053	8,596
Assets, earnings	363	2,005
Assets, other comprehensive income	0
Assets, impairments	0	0
Assets, purchases	0	0
Assets, (sales)	(1,066)	(1,548)
Assets. transfers out	0	0
Assets, ending balance	8,350	9,053
Level 3 | Preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	6,505	3,945
Assets, earnings	(177)	(42)
Assets, other comprehensive income	0	0
Assets, impairments	0	0
Assets, purchases	3,000	2,602
Assets, (sales)	0	0
Assets. transfers out	0	0
Assets, ending balance	9,328	6,505
Level 3 | Arbitrage trading account
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	$ 0	17,308
Assets, earnings		(8,731)
Assets, other comprehensive income		0
Assets, impairments		0
Assets, purchases		14,767
Assets, (sales)		(38,233)
Assets. transfers out		14,889
Assets, ending balance		$ 0