Cash Equivalent and Investments (Parenthetical) (Detail)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Minimum [Member] | Certificates Of Deposit [Member] | Short Term Investments [Member]
Schedule Of Investments [Line Items]
Securities Maturity period	1 month	1 month
Minimum [Member] | Certificates Of Deposit [Member] | Other Assets [Member]
Schedule Of Investments [Line Items]
Securities Maturity period	14 months	15 months
Minimum [Member] | Fixed Maturity and Mutual Funds [Member]
Schedule Of Investments [Line Items]
Securities Maturity period	3 months	6 months
Maximum [Member] | Certificates Of Deposit [Member] | Short Term Investments [Member]
Schedule Of Investments [Line Items]
Securities Maturity period	12 months	12 months
Maximum [Member] | Certificates Of Deposit [Member] | Cash And Cash Equivalents [Member]
Schedule Of Investments [Line Items]
Securities Maturity period	90 days
Maximum [Member] | Certificates Of Deposit [Member] | Other Assets [Member]
Schedule Of Investments [Line Items]
Securities Maturity period	45 months	48 months
Maximum [Member] | Fixed Maturity and Mutual Funds [Member]
Schedule Of Investments [Line Items]
Securities Maturity period	4 months	7 months