Reinsurance - Industry pools and facilities (Details)	6 Months Ended	12 Months Ended				24 Months Ended
	Jun. 30, 2017 USD ($)	May 31, 2018 USD ($) hurricane	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2017 USD ($)
Reinsurance Retention Policy [Line Items]
Reinsurance recoverables, net			$ 8,921,000,000	$ 8,745,000,000
Cash and invested assets			112,422,000,000	108,610,000,000
Accumulated deficit			(43,162,000,000)	(40,678,000,000)
Ceded premium earned			1,274,000,000	1,296,000,000	$ 1,338,000,000
Michigan Catastrophic Claim Association
Reinsurance Retention Policy [Line Items]
Reinsurance recoverables, net			$ 5,260,000,000	4,950,000,000
Reinsurance increase, percentage			6.00%
Retention level per claim							$ 545,000
Term of catastrophe reinsurance agreement contract							2 years
Program funding, per vehicle annual assessment			$ 170
Cash and invested assets	$ 19,600,000,000						$ 19,600,000,000
Accumulated deficit	2,630,000,000						$ 2,630,000,000
Retained earnings accumulated deficit increase (decrease)	$ 46,080,000,000
National Flood Insurance Program
Reinsurance Retention Policy [Line Items]
Reinsurance recoverables, net			88,000,000	77,000,000
Ceded premium earned			263,000,000	274,000,000	293,000,000
Ceded losses incurred			1,120,000,000	537,000,000	120,000,000
New Jersey Unsatisfied Claim and Judgment Fund
Reinsurance Retention Policy [Line Items]
Reinsurance recoverables, net			493,000,000	506,000,000
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			8,900,000
Florida Hurricane Catastrophe Fund
Reinsurance Retention Policy [Line Items]
Reinsurance recoverables, net			19,000,000	0
Ceded premium earned			11,000,000	12,000,000	13,000,000
Ceded losses incurred			$ 19,000,000	$ 0	$ 0
Scenario, Forecast | Michigan Catastrophic Claim Association
Reinsurance Retention Policy [Line Items]
Retention level per claim						$ 555,000
Term of catastrophe reinsurance agreement contract						2 years
Scenario, Forecast | Florida Hurricane Catastrophe Fund
Reinsurance Retention Policy [Line Items]
Percent of losses in excess of retention level to be reimbursed		90.00%
Retention level for losses related to 2 largest hurricanes		$ 56,000,000
Number of storms | hurricane		2
Retention level for losses related to other hurricanes		$ 19,000,000
Maximum Retention level for losses related to hurricanes		$ 187,000,000