Statutory Accounting Practices - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Statutory Accounting Practices [Line Items]
Increase in statutory capital and surplus	$ 63	$ 67
Combined Continental Casualty Companies [Member]
Statutory Accounting Practices [Line Items]
Increase in statutory capital and surplus	10,726	$ 10,748
Dividends payable without prior supervisory approval	1,100
Dividends paid	$ 955
Statutory capital and surplus percentage	264.00%	270.00%