Significant Accounting Policies (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
Schedule Of Significant Accounting Policies [Line Items]
Decrease to retained earnings	$ (587,867,000)		$ (573,619,000)
Accrued interest receivable on available-for-sale securities	5,100,000
Premium receivable, allowance for credit loss	800,000		700,000
Reinsurance, allowance for credit loss	$ 0		$ 0
Cumulative Effect, Period Of Adoption, Adjustment [Member] | Accounting Standards Update 2016-13 [Member]
Schedule Of Significant Accounting Policies [Line Items]
Decrease to retained earnings		$ 600,000
Minimum [Member]
Schedule Of Significant Accounting Policies [Line Items]
Short-term investment maturity period	3 months