Fair Value Measurements (Summarize Changes in Level 3 Assets) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
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	20,385	12,599
Assets, earnings	(475)	19
Assets, other comprehensive income	46	650
Assets, purchases	3,882	7,173
Assets, (sales)	(126)	(56)
Assets. transfers out	(3,842)	0
Assets, ending balance	19,870
Level 3 | Debt securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	172	183
Assets, earnings	(2)	3
Assets, other comprehensive income	46	34
Assets, impairments	0	0
Assets, purchases	0	0
Assets, (sales)	(115)	(48)
Assets. transfers out	0	0
Assets, ending balance	101
Level 3 | Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	172	183
Assets, earnings	(2)	3
Assets, other comprehensive income	46	34
Assets, impairments	0	0
Assets, purchases	0	0
Assets, (sales)	(115)	(48)
Assets, maturities	0	0
Assets. transfers out	0	0
Assets, ending balance	101
Level 3 | Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	20,213	12,416
Assets, earnings	(444)	8
Assets, other comprehensive income	0	616
Assets, impairments	0	0
Assets, purchases	0	7,173
Assets, (sales)	0	0
Assets. transfers out	0	0
Assets, ending balance	19,769
Level 3 | Common stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	9,370	8,754
Assets, earnings	(445)	0
Assets, other comprehensive income	0	616
Assets, impairments	0	0
Assets, purchases	0	0
Assets, (sales)	0	0
Assets. transfers out	0	0
Assets, ending balance	8,925
Level 3 | Preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	10,843	3,662
Assets, earnings	1	8
Assets, other comprehensive income	0	0
Assets, impairments	0	0
Assets, purchases	0	7,173
Assets, (sales)	0	0
Assets. transfers out	0	0
Assets, ending balance	10,844
Level 3 | Arbitrage trading account
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	0	0
Assets, earnings	(29)	8
Assets, other comprehensive income	0	0
Assets, impairments	0	0
Assets, purchases	3,882	0
Assets, (sales)	(11)	(8)
Assets. transfers out	3,842	$ 0
Assets, ending balance	$ 0