Investments - Unrealized Gains or (Losses) Included in Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gains/Losses included in Other Comprehensive Income for Assets Held at the End of the Period	$ 14,386	$ 32,600	$ (21,108)
Asset- backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gains/Losses included in Other Comprehensive Income for Assets Held at the End of the Period	(173)	708	(591)
Collateralized debt obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gains/Losses included in Other Comprehensive Income for Assets Held at the End of the Period	(2,523)	1,514	3,170
Corporates
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gains/Losses included in Other Comprehensive Income for Assets Held at the End of the Period	$ 17,082	$ 30,378	$ (23,687)