Commitments and Contingencies - Additional Information (Details)	1 Months Ended		3 Months Ended	12 Months Ended
	Mar. 31, 2016 plaintiff	Feb. 28, 2011 plaintiff	Mar. 31, 2018 USD ($)	Dec. 31, 2016 USD ($)
Asbestos and Environmental
Loss Contingencies [Line Items]
Reinsurance premium				$ 650,000,000
Maximum adverse loss development to be ceded under agreement				1,500,000,000.0
Asbestos
Loss Contingencies [Line Items]
Liability for unpaid losses and loss adjustment expenses, at undiscounted amounts			$ 1,200,000,000
Environmental
Loss Contingencies [Line Items]
Liability for unpaid losses and loss adjustment expenses, at undiscounted amounts			231,000,000
Asbestos and Environmental
Loss Contingencies [Line Items]
Liability for unpaid losses and loss adjustment expenses, at undiscounted amounts				$ 1,700,000,000
Adverse development from comprehensive annual review			285,000,000
Settled Litigation [Member] | Mutual Funds Litigation [Member]
Loss Contingencies [Line Items]
Number of funds on which action has been brought | plaintiff	1
Pending Litigation [Member] | Mutual Funds Litigation [Member]
Loss Contingencies [Line Items]
Number of funds on which action has been brought | plaintiff	6	6
Maximum | Asbestos and Environmental
Loss Contingencies [Line Items]
Adverse development from comprehensive annual review			$ 0