Fair Value - Quantitative and Qualitative Disclosures about Unobservable Inputs (Details) $ in Thousands	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 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	$ 14,065	$ 8,924	$ 12,577	$ 9,175	$ 11,120	$ 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	$ 4,707	$ 2,551	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)	(0.0536)
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)	0.0035
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)	0.129
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)	0.0083
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)	0.0087
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)	0.0057
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	$ 2,400		$ 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 | Available-for-sale securities
Fair value measurements on a recurring basis, valuation techniques
Fair value, measurement with unobservable inputs	6,827
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	$ 4,707