Fair Value Measurement - Quantitative Information Regarding Level 3 Valuations (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets:
Fixed maturities, available for sale	$ 2,288,785	$ 2,093,798
Fair Value, Inputs, Level 3 | Corporate debt, limited observable inputs
Assets:
Fixed maturities, available for sale	$ 5,079	4,322
Fair Value, Inputs, Level 3 | Corporate debt, limited observable inputs | Market Comparable Securities | Comparability Adjustment | Minimum
Assets:
Fixed maturities, available-for-sale, measurement input	0.00
Fair Value, Inputs, Level 3 | Corporate debt, limited observable inputs | Market Comparable Securities | Comparability Adjustment | Maximum
Assets:
Fixed maturities, available-for-sale, measurement input	0.05
Fair Value, Inputs, Level 3 | Corporate debt, limited observable inputs | Market Comparable Securities | Comparability Adjustment | Weighted Average
Assets:
Fixed maturities, available-for-sale, measurement input	0.025
Fair Value, Inputs, Level 3 | Corporate debt, limited observable inputs | Discounted Cash Flows | Comparability Adjustment | Minimum
Assets:
Fixed maturities, available-for-sale, measurement input	0.00
Fair Value, Inputs, Level 3 | Corporate debt, limited observable inputs | Discounted Cash Flows | Comparability Adjustment | Maximum
Assets:
Fixed maturities, available-for-sale, measurement input	0.05
Fair Value, Inputs, Level 3 | Corporate debt, limited observable inputs | Discounted Cash Flows | Comparability Adjustment | Weighted Average
Assets:
Fixed maturities, available-for-sale, measurement input	0.025
Fair Value, Inputs, Level 3 | Other asset-backed securities
Assets:
Fixed maturities, available for sale	$ 2,992	3,850
Fair Value, Inputs, Level 3 | Other asset-backed securities | Market Comparable Securities | Comparability Adjustment | Minimum
Assets:
Fixed maturities, available-for-sale, measurement input	0.00
Fair Value, Inputs, Level 3 | Other asset-backed securities | Market Comparable Securities | Comparability Adjustment | Maximum
Assets:
Fixed maturities, available-for-sale, measurement input	0.05
Fair Value, Inputs, Level 3 | Other asset-backed securities | Market Comparable Securities | Comparability Adjustment | Weighted Average
Assets:
Fixed maturities, available-for-sale, measurement input	0.025
Fair Value, Inputs, Level 3 | Other asset-backed securities | Discounted Cash Flows | Comparability Adjustment | Minimum
Assets:
Fixed maturities, available-for-sale, measurement input	0.00
Fair Value, Inputs, Level 3 | Other asset-backed securities | Discounted Cash Flows | Comparability Adjustment | Maximum
Assets:
Fixed maturities, available-for-sale, measurement input	0.05
Fair Value, Inputs, Level 3 | Other asset-backed securities | Discounted Cash Flows | Comparability Adjustment | Weighted Average
Assets:
Fixed maturities, available-for-sale, measurement input	0.025
Fair Value, Inputs, Level 3 | Other investments
Assets:
Other investments	$ 3,086	$ 3
Fair Value, Inputs, Level 3 | Other investments | Discounted Cash Flows | Comparability Adjustment | Minimum
Assets:
Other investments, measurement input	0.00%
Fair Value, Inputs, Level 3 | Other investments | Discounted Cash Flows | Comparability Adjustment | Maximum
Assets:
Other investments, measurement input	10.00%
Fair Value, Inputs, Level 3 | Other investments | Discounted Cash Flows | Comparability Adjustment | Weighted Average
Assets:
Other investments, measurement input	5.00%