Components of Net Periodic Benefit Cost (Narrative) (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2006	Dec. 31, 2002
Retirement Benefits [Abstract]
Service period for employees of coverage age, years (at least)	1 year
Defined benefit plan, percentage vested	100.00%
Retiree benefits eliminated if less than service period, years (less than)		10 years	10 years
Defined benefit plan, contributions by employer	$ 0