Investments - Unrealized Gains or (Losses) Included in Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
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	$ (39,823)	$ 15,137
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	(276)	298
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	(5,475)	4,176
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	$ (34,072)	$ 10,663