Fair Value Measurement (Quantitative Information Regarding Level 3 Valuations) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets:
Estimated Fair Value	$ 2,386,085	$ 2,288,785
Corporate debt, limited observable inputs | Level 3
Assets:
Estimated Fair Value	$ 3,995	5,079
Corporate debt, limited observable inputs | Level 3 | Comparability Adjustment | Market Comparable Securities | Minimum
Assets:
Available-for-sale, measurement input	0.00
Corporate debt, limited observable inputs | Level 3 | Comparability Adjustment | Market Comparable Securities | Maximum
Assets:
Available-for-sale, measurement input	0.05
Corporate debt, limited observable inputs | Level 3 | Comparability Adjustment | Market Comparable Securities | Weighted Avg
Assets:
Available-for-sale, measurement input	0.025
Corporate debt, limited observable inputs | Level 3 | Comparability Adjustment | Discounted Cash Flows | Minimum
Assets:
Available-for-sale, measurement input	0.00
Corporate debt, limited observable inputs | Level 3 | Comparability Adjustment | Discounted Cash Flows | Maximum
Assets:
Available-for-sale, measurement input	0.05
Corporate debt, limited observable inputs | Level 3 | Comparability Adjustment | Discounted Cash Flows | Weighted Avg
Assets:
Available-for-sale, measurement input	0.025
Residential mortgage-backed securities | Level 3
Assets:
Estimated Fair Value	$ 4,120	0
Residential mortgage-backed securities | Level 3 | Comparability Adjustment | Market Comparable Securities | Minimum
Assets:
Available-for-sale, measurement input	0.00
Residential mortgage-backed securities | Level 3 | Comparability Adjustment | Market Comparable Securities | Maximum
Assets:
Available-for-sale, measurement input	0.05
Residential mortgage-backed securities | Level 3 | Comparability Adjustment | Market Comparable Securities | Weighted Avg
Assets:
Available-for-sale, measurement input	0.025
Residential mortgage-backed securities | Level 3 | Comparability Adjustment | Discounted Cash Flows | Minimum
Assets:
Available-for-sale, measurement input	0.00
Residential mortgage-backed securities | Level 3 | Comparability Adjustment | Discounted Cash Flows | Maximum
Assets:
Available-for-sale, measurement input	0.05
Residential mortgage-backed securities | Level 3 | Comparability Adjustment | Discounted Cash Flows | Weighted Avg
Assets:
Available-for-sale, measurement input	0.025
Other asset-backed securities | Level 3
Assets:
Estimated Fair Value	$ 8,834	2,992
Other asset-backed securities | Level 3 | Comparability Adjustment | Market Comparable Securities | Minimum
Assets:
Available-for-sale, measurement input	0.00
Other asset-backed securities | Level 3 | Comparability Adjustment | Market Comparable Securities | Maximum
Assets:
Available-for-sale, measurement input	0.05
Other asset-backed securities | Level 3 | Comparability Adjustment | Market Comparable Securities | Weighted Avg
Assets:
Available-for-sale, measurement input	0.025
Other asset-backed securities | Level 3 | Comparability Adjustment | Discounted Cash Flows | Minimum
Assets:
Available-for-sale, measurement input	0.00
Other asset-backed securities | Level 3 | Comparability Adjustment | Discounted Cash Flows | Maximum
Assets:
Available-for-sale, measurement input	0.05
Other asset-backed securities | Level 3 | Comparability Adjustment | Discounted Cash Flows | Weighted Avg
Assets:
Available-for-sale, measurement input	0.025
Other investments | Level 3
Assets:
Assets measured at fair value	$ 0	$ 3,086
Other investments | Level 3 | Comparability Adjustment | Discounted Cash Flows | Minimum
Assets:
Other assets, measurement input	0.00%
Other investments | Level 3 | Comparability Adjustment | Discounted Cash Flows | Maximum
Assets:
Other assets, measurement input	10.00%
Other investments | Level 3 | Comparability Adjustment | Discounted Cash Flows | Weighted Avg
Assets:
Other assets, measurement input	5.00%