Cash Equivalent and Investments (Parenthetical) (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
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	13 months	13 months
Minimum [Member] | Fixed Maturity and Mutual Funds [Member]
Schedule Of Investments [Line Items]
Securities Maturity period	9 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	51 months	15 months
Maximum [Member] | Fixed Maturity and Mutual Funds [Member]
Schedule Of Investments [Line Items]
Securities Maturity period	10 months	19 months