Fair Value Measurements (Summarize Changes in Level 3 Assets) (Details) - USD ($)	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, impairments	$ 0	$ 0
Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	29,948,000	20,385,000	$ 20,385,000
Assets, earnings	(8,765,000)	(456,000)	0
Assets, other comprehensive income	1,868,000	46,000
Assets, purchases	17,369,000	11,523,000
Assets, (sales)	(39,915,000)	(7,353,000)
Assets. transfers out	14,889,000	5,803,000
Assets, ending balance	15,394,000		29,948,000
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities, beginning balance	793,000	0	0
Liabilities, earnings	133,000		(67,000)
Liabilities, other comprehensive income	0		0
Liabilities, impairments	0
Liabilities, purchases	7,609,000		860,000
Liabilities, (sales)	(8,535,000)		0
Liabilities, maturities	0		0
Liabilities, ending balance	0		793,000
Level 3 | Debt securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	99,000	172,000	172,000
Assets, earnings	(26,000)	(2,000)
Assets, other comprehensive income	61,000	46,000
Assets, impairments	0	0
Assets, purchases	0	0
Assets, (sales)	(134,000)	(117,000)
Assets. transfers out	0	0
Assets, ending balance	0		99,000
Level 3 | Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	99,000	172,000	172,000
Assets, earnings	(26,000)	(2,000)
Assets, other comprehensive income	61,000	46,000
Assets, impairments	0	0
Assets, purchases	0	0
Assets, (sales)	(134,000)	(117,000)
Assets, maturities	0	0
Assets. transfers out	0	0
Assets, ending balance	0		99,000
Level 3 | Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	12,541,000	20,213,000	20,213,000
Assets, earnings	(8,000)	(448,000)
Assets, other comprehensive income	1,807,000	0
Assets, impairments	0	0
Assets, purchases	2,602,000	0
Assets, (sales)	(1,548,000)	(7,225,000)
Assets, maturities		0
Assets. transfers out	0	1,000
Assets, ending balance	15,394,000		12,541,000
Level 3 | Common stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	8,596,000	9,370,000	9,370,000
Assets, earnings	12,000	(548,000)
Assets, other comprehensive income	1,807,000	0
Assets, impairments	0	0
Assets, purchases	0	0
Assets, (sales)	(1,548,000)	(227,000)
Assets. transfers out	0	1,000
Assets, ending balance	8,867,000		8,596,000
Level 3 | Preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	3,945,000	10,843,000	10,843,000
Assets, earnings	(20,000)	100,000
Assets, other comprehensive income	0	0
Assets, impairments	0	0
Assets, purchases	2,602,000
Assets, (sales)	0	(6,998,000)
Assets. transfers out	0	0
Assets, ending balance	6,527,000		3,945,000
Level 3 | Arbitrage trading account
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	17,308,000	0	0
Assets, earnings	(8,731,000)	(6,000)
Assets, other comprehensive income	0	0
Assets, impairments	0	0
Assets, purchases	14,767,000	11,523,000
Assets, (sales)	(38,233,000)	(11,000)
Assets. transfers out	14,889,000	$ 5,802,000
Assets, ending balance	0		17,308,000
Level 3 | Securities Sold, Not yet Purchased [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets. transfers out			$ 0
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities, transfers out	$ 0