Reinsurance and Indemnification - Industry pools and facilities (Details) - USD ($)	6 Months Ended	12 Months Ended			24 Months Ended
	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2019
Reinsurance Retention Policy [Line Items]
Cash and invested assets		$ 119,950,000,000	$ 112,249,000,000
Accumulated deficit		(48,074,000,000)	(44,033,000,000)
Ceded premium earned		1,407,000,000	1,306,000,000	$ 1,274,000,000
Michigan Catastrophic Claim Association
Reinsurance Retention Policy [Line Items]
Reinsurance and indemnification recoverables, net		5,500,000,000	5,400,000,000
Program funding, per vehicle annual assessment		$ 220
Reinsurance increase, percentage		6.00%
Retention level per claim					$ 555,000
Cash and invested assets	$ 21,830,000,000				21,830,000,000
Accumulated deficit	(1,280,000,000)				$ (1,280,000,000)
Retained earnings accumulated deficit decrease	$ 39,640,000,000
New Jersey Unsatisfied Claim and Judgment Fund
Reinsurance Retention Policy [Line Items]
Reinsurance and indemnification recoverables, net		$ 446,000,000	$ 461,000,000
Minimum medical benefits portion of personal injury protection coverage for reimbursement to insurers for policies issued or renewed prior to January 1, 1991		75,000
Minimum medical benefits portion of personal injury protection coverage for reimbursement to insurers for policies issued or renewed from January 1, 1991 to December 31, 2004		75,000
Maximum reimbursement to insurers for medical benefits paid under personal injury protection coverage.		250,000
Reinsurance, administration assessments paid		$ 8,100,000
Florida Hurricane Catastrophe Fund
Reinsurance Retention Policy [Line Items]
Emergency assessment maximum percentage of premiums first year		6.00%
Emergency assessment maximum percentage of premiums subsequent years		10.00%
Emergency assessment maximum percentage of premiums subsequent years, FL OIR		0.00%
Emergency assessment, bonds issued				$ 2,000,000,000.00