Fair Value - Quantitative and Qualitative Disclosures about Unobservable Inputs (Details) $ in Thousands	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Level 3 | Available-for-sale securities
Fair value measurements on a recurring basis, valuation techniques
Fair value, measurement with unobservable inputs	$ 11,645	$ 12,577	$ 10,079
Level 3 | Commercial mortgage-backed securities | Available-for-sale securities
Fair value measurements on a recurring basis, valuation techniques
Fair value, measurement with unobservable inputs	$ 3,321	0
Minimum | Valuation Technique, Syndicated Loan Model | Corporate debt securities - bank loans
Fair value measurements on a recurring basis, valuation techniques
Debt instrument, measurement input (percent)	(0.0489)
Minimum | Valuation Technique, Relative Value Pricing Model | Commercial mortgage-backed securities
Fair value measurements on a recurring basis, valuation techniques
Debt instrument, measurement input (percent)	0.0049
Maximum | Valuation Technique, Syndicated Loan Model | Corporate debt securities - bank loans
Fair value measurements on a recurring basis, valuation techniques
Debt instrument, measurement input (percent)	0.0566
Maximum | Valuation Technique, Relative Value Pricing Model | Commercial mortgage-backed securities
Fair value measurements on a recurring basis, valuation techniques
Debt instrument, measurement input (percent)	0.0053
Weighted Average | Valuation Technique, Syndicated Loan Model | Corporate debt securities - bank loans
Fair value measurements on a recurring basis, valuation techniques
Debt instrument, measurement input (percent)	0.0076
Weighted Average | Valuation Technique, Relative Value Pricing Model | Commercial mortgage-backed securities
Fair value measurements on a recurring basis, valuation techniques
Debt instrument, measurement input (percent)	0.0051
Fair Value, Measurements, Recurring Basis | Valuation, Held At Cost | Level 3 | Available-for-sale securities
Fair value measurements on a recurring basis, valuation techniques
Fair value, measurement with unobservable inputs	$ 200
Fair Value, Measurements, Recurring Basis | Valuation, Non-Binding Broker Quote | Level 3 | Available-for-sale securities
Fair value measurements on a recurring basis, valuation techniques
Fair value, measurement with unobservable inputs	1,300	$ 12,600
Fair Value, Measurements, Recurring Basis | Valuation Technique, Syndicated Loan Model | Level 3 | Corporate debt securities - bank loans | Available-for-sale securities
Fair value measurements on a recurring basis, valuation techniques
Fair value, measurement with unobservable inputs	8,048
Fair Value, Measurements, Recurring Basis | Valuation Technique, Relative Value Pricing Model | Level 3 | Commercial mortgage-backed securities | Available-for-sale securities
Fair value measurements on a recurring basis, valuation techniques
Fair value, measurement with unobservable inputs	$ 2,143