Fair Value Measurements (Summarize Changes in Level 3 Assets) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	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	$ 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	793
Liabilities, earnings			133
Liabilities, other comprehensive income			0
Liabilities, impairments			0
Liabilities, purchases			7,609
Liabilities, (sales)			(8,535)
Liabilities, maturities			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	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	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	12,541
Assets, earnings	880	1,963
Assets, other comprehensive income	0	0
Assets, impairments	0	0
Assets, purchases	0	2,602
Assets, (sales)	(1,066)	(1,548)
Assets, maturities		0
Assets. transfers out	0	0
Assets, ending balance	15,372		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	8,596
Assets, earnings	880	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	8,867		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	3,945
Assets, earnings	0	(42)
Assets, other comprehensive income	0	0
Assets, impairments	0	0
Assets, purchases	0	2,602
Assets, (sales)	0	0
Assets. transfers out	0	0
Assets, ending balance	6,505		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	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
Level 3 | Securities Sold, Not yet Purchased [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities, transfers out			$ 0