Fair Value - Quantitative and Qualitative Disclosures about Unobservable Inputs (Details) $ in Thousands	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 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	$ 8,924	$ 13,620	$ 12,577	$ 11,120	$ 6,309	$ 10,079
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	6,373	11,523	12,577	$ 11,120	$ 6,309	$ 7,879
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	$ 2,551	$ 1,182	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)	(0.013)
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)	0.0044
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)	0.073
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)	0.0052
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)	0.0054
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)	0.0048
Fair Value, Measurements, Recurring Basis | Level 3 | Available-for-sale securities
Fair value measurements on a recurring basis, valuation techniques
Fair value, measurement with unobservable inputs	$ 800		$ 12,600
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 Technique, Syndicated Loan Model | Level 3 | Corporate debt securities - bank loans
Fair value measurements on a recurring basis, valuation techniques
Fair value, measurement with unobservable inputs	6,093
Fair Value, Measurements, Recurring Basis | Valuation Technique, Relative Value Pricing Model | Level 3 | Commercial mortgage-backed securities
Fair value measurements on a recurring basis, valuation techniques
Fair value, measurement with unobservable inputs	$ 1,866