Reinsurance and Indemnification - Per Occurrence Excess Agreement (Details) $ in Millions	9 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($) state layer reinstatement	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]
Retention percentage of reinsurance limit		100.00%	100.00%
Limit of aggregate losses under aggregate excess agreement	$ 500	$ 500	$ 500
Number of states covered by reinsurance agreement | state		49
Term of catastrophe reinsurance agreement contract		4 years
Catastrophe Reinsurance First Five Layers
Reinsurance Retention Policy [Line Items]
Number of layers subject to reinstatement in which catastrophe reinsurance agreement contracts are placed | layer		3
Retention percentage of reinsurance limit		95.00%
Catastrophe Reinsurance Sixth Layer
Reinsurance Retention Policy [Line Items]
Retention percentage of reinsurance limit		95.00%
Limit of aggregate losses under aggregate excess agreement		$ 324
Term of catastrophe reinsurance agreement contract		7 years
Catastrophe Reinsurance Seventh Layer
Reinsurance Retention Policy [Line Items]
Retention percentage of reinsurance limit		29.37%
Retention amount for catastrophe reinsurance agreement contracts		$ 446
Number of reinstatements each year for each contract | reinstatement		1
Minimum | 2018-1 Excess Catastrophe Reinsurance Contract
Reinsurance Retention Policy [Line Items]
Retention amount for catastrophe reinsurance agreement contracts		$ 3,940	$ 4,360