Reinsurance and Indemnification - 2018-1 Excess Catastrophe Reinsurance Contract (Details) $ in Millions	9 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($) state	Mar. 31, 2019 USD ($)
Reinsurance Retention Policy [Line Items]
Limit of aggregate losses under aggregate excess agreement		$ 3,400
2018-1 Excess Catastrophe Reinsurance Contract
Reinsurance Retention Policy [Line Items]
Number of states covered under reinsurance agreement | state		49
Limit of aggregate losses under aggregate excess agreement	$ 500	$ 500	$ 500
Term of catastrophe reinsurance agreement contract		4 years
Retention percentage of reinsurance limit		100.00%	100.00%
Minimum | 2018-1 Excess Catastrophe Reinsurance Contract
Reinsurance Retention Policy [Line Items]
Retention amount for catastrophe reinsurance agreement contracts		$ 3,940	$ 4,360
Maximum | 2018-1 Excess Catastrophe Reinsurance Contract
Reinsurance Retention Policy [Line Items]
Retention amount for catastrophe reinsurance agreement contracts		$ 4,440	$ 4,860