Fair Value Measurements (Summarize Changes in Level 3 Assets) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
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	12,599	11,982
Assets, earnings	29	42
Assets, other comprehensive income	665	176
Assets, purchases	175	765
Assets, (sales)	(51)	(331)
Assets, maturities		(35)
Assets. transfers out		0
Assets, ending balance	13,417	12,599
Level 3 | Debt securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	183	353
Assets, earnings	2	180
Assets, other comprehensive income	32	16
Assets, impairments	0	0
Assets, purchases	0	0
Assets, (sales)	(43)	(331)
Assets, maturities		(35)
Assets. transfers out	0	0
Assets, ending balance	174	183
Level 3 | Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	183	199
Assets, earnings	2	3
Assets, other comprehensive income	32	16
Assets, impairments	0	0
Assets, purchases	0	0
Assets, (sales)	(43)	0
Assets, maturities	0	(35)
Assets. transfers out	0	0
Assets, ending balance	174	183
Level 3 | Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	0	154
Assets, earnings	0	177
Assets, other comprehensive income	0	0
Assets, impairments	0	0
Assets, purchases	0	0
Assets, (sales)	0	(331)
Assets. transfers out	0	0
Assets, ending balance	0	0
Level 3 | Equity securities available for sale
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	12,416	11,453
Assets, earnings	19	38
Assets, other comprehensive income	633	160
Assets, impairments	0	0
Assets, purchases	175	765
Assets, (sales)	0	0
Assets. transfers out	0	0
Assets, ending balance	13,243	12,416
Level 3 | Common stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	8,754	7,829
Assets, earnings	0	0
Assets, other comprehensive income	633	160
Assets, impairments	0	0
Assets, purchases	0	765
Assets, (sales)	0	0
Assets. transfers out	0	0
Assets, ending balance	9,387	8,754
Level 3 | Preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	3,662	3,624
Assets, earnings	19	38
Assets, other comprehensive income	0	0
Assets, impairments	0	0
Assets, purchases	175	0
Assets, (sales)	0	0
Assets. transfers out	0	0
Assets, ending balance	3,856	3,662
Level 3 | Arbitrage trading account
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, beginning balance	0	176
Assets, earnings	8	(176)
Assets, other comprehensive income	0	0
Assets, impairments	0	0
Assets, purchases	0	0
Assets, (sales)	(8)	0
Assets. transfers out	0	0
Assets, ending balance	$ 0	$ 0