Recent Accounting Developments - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Reclassification from accumulated other comprehensive income to retained earnings	$ 17,000,000
ASU 2016-02 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Lessee, operating lease term		12 months
ASU 2016-02 [Member] | Minimum [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Recognizing lease liabilities	385,000,000
ASU 2016-02 [Member] | Maximum [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Recognizing lease liabilities	$ 410,000,000