Fair Value Measurement (Quantitative Information Regarding Level 3 Valuations) (Details) - Level 3 - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Corporate debt, limited observable inputs
Assets:
Fair Value at	$ 8,380	$ 13,703
Corporate debt, limited observable inputs | Market Comparable Securities | Minimum
Assets:
Comparability Adjustment	0.00%
Corporate debt, limited observable inputs | Market Comparable Securities | Maximum
Assets:
Comparability Adjustment	5.00%
Corporate debt, limited observable inputs | Market Comparable Securities | Weighted Average
Assets:
Comparability Adjustment	2.50%
Corporate debt, limited observable inputs | Discounted Cash Flows | Minimum
Assets:
Comparability Adjustment	0.00%
Corporate debt, limited observable inputs | Discounted Cash Flows | Maximum
Assets:
Comparability Adjustment	5.00%
Corporate debt, limited observable inputs | Discounted Cash Flows | Weighted Average
Assets:
Comparability Adjustment	2.50%
Residential mortgage-backed and other asset-backed securities
Assets:
Fair Value at	$ 9,420	4,986
Residential mortgage-backed and other asset-backed securities | Market Comparable Securities | Minimum
Assets:
Comparability Adjustment	0.00%
Residential mortgage-backed and other asset-backed securities | Market Comparable Securities | Maximum
Assets:
Comparability Adjustment	5.00%
Residential mortgage-backed and other asset-backed securities | Market Comparable Securities | Weighted Average
Assets:
Comparability Adjustment	2.50%
Residential mortgage-backed and other asset-backed securities | Discounted Cash Flows | Minimum
Assets:
Comparability Adjustment	0.00%
Residential mortgage-backed and other asset-backed securities | Discounted Cash Flows | Maximum
Assets:
Comparability Adjustment	5.00%
Residential mortgage-backed and other asset-backed securities | Discounted Cash Flows | Weighted Average
Assets:
Comparability Adjustment	2.50%
Other investments
Assets:
Fair Value at	$ 5	$ 409
Other investments | Discounted Cash Flows | Minimum
Assets:
Comparability Adjustment	0.00%
Other investments | Discounted Cash Flows | Maximum
Assets:
Comparability Adjustment	10.00%
Other investments | Discounted Cash Flows | Weighted Average
Assets:
Comparability Adjustment	5.00%