Reinsurance - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Reinsurance recoverables, net	$ 4,061,000,000	$ 3,659,000,000
Unpaid Losses and Loss Adjustment Expenses	32,287,000,000	28,317,000,000
Ceded losses	901,000,000	388,000,000	$ 336,000,000
Asbestos and Environmental
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Reinsurance premium		650,000,000
Maximum adverse loss development to be ceded under agreement		1,500,000,000.0
Unpaid Losses and Loss Adjustment Expenses	2,217,000,000	1,700,000,000
Accrued reinsurance premium		$ 650,000,000
Adverse reserve development ceded	$ 285,000,000