Income Taxes - Principal Components of Deferred Tax Assets and Liabilities (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components Of Deferred Income Tax Assets And Liabilities [Line Items]
U.S. federal statutory rate		35.00%	34.00%	34.00%
Scenario, Plan [Member]
Components Of Deferred Income Tax Assets And Liabilities [Line Items]
U.S. federal statutory rate	21.00%