Fair Values of Financial Instruments - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Jan. 19, 2018	Jun. 14, 2012	Oct. 05, 2010
3.95% Senior Notes Due 2028
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt interest rate	3.95%	3.95%	3.95%
Current market rates used in measuring fair value	4.65%	5.07%
4.97% Series A Senior Notes, Due October 5th 2020
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt interest rate					4.97%
Current market rates used in measuring fair value	4.63%	4.97%
4.27% Series B Senior Notes, Due June 14th 2022
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt interest rate				4.27%
Current market rates used in measuring fair value	4.51%	4.92%