Commitments and Contingencies - Additional Information (Details) $ in Millions	1 Months Ended		6 Months Ended	12 Months Ended
	Mar. 31, 2016 plaintiff	Feb. 28, 2011 plaintiff	Jun. 30, 2018 USD ($)	Dec. 31, 2016 USD ($)
Asbestos and Environmental
Loss Contingencies [Line Items]
Maximum adverse loss development to be ceded under agreement				$ 1,500
Reinsurance premium				650
Asbestos
Loss Contingencies [Line Items]
Liability for unpaid losses and loss adjustment expenses, at undiscounted amounts			$ 1,100
Environmental
Loss Contingencies [Line Items]
Liability for unpaid losses and loss adjustment expenses, at undiscounted amounts			223
Asbestos and Environmental
Loss Contingencies [Line Items]
Liability for unpaid losses and loss adjustment expenses, at undiscounted amounts				$ 1,700
Adverse development from comprehensive annual review			285
Asbestos and Environmental | Maximum
Loss Contingencies [Line Items]
Adverse development from comprehensive annual review			$ 1,200
Pending Litigation [Member] | Mutual Funds Litigation [Member]
Loss Contingencies [Line Items]
Number of funds on which action has been brought | plaintiff	6	6
Settled Litigation [Member] | Mutual Funds Litigation [Member]
Loss Contingencies [Line Items]
Number of funds on which action has been brought | plaintiff	1