Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2019	Dec. 31, 2016
Asbestos and Environmental
Loss Contingencies [Line Items]
Reinsurance premium			$ 650
Asbestos and Environmental | Maximum
Loss Contingencies [Line Items]
Maximum adverse loss development to be ceded under agreement		$ 1,500
Asbestos
Loss Contingencies [Line Items]
Liability for unpaid losses and loss adjustment expenses, at undiscounted amounts	$ 1,000	1,000
Environmental
Loss Contingencies [Line Items]
Liability for unpaid losses and loss adjustment expenses, at undiscounted amounts	196	196
Asbestos and Environmental
Loss Contingencies [Line Items]
Maximum adverse loss development to be ceded under agreement	$ 977
Reinsurance Retention Policy, Amount Retained		1,700
Reinsurance Retention Policy, Excess Retention, Amount Reinsured		$ 523