Fair Value Measurement of Assets and Liabilities - Changes in Level 3 Assets Measured at Fair Value on Recurring Basis (Detail) (Available-for-sale Securities [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Available-for-sale Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance, beginning of the period	$ 19,309	$ 28,523
Total net (losses) gains included in other comprehensive income for the period	(792)	48
Sales	(2,675)	0
Settlements	(374)	(1,660)
Balance, end of the period	$ 15,468	$ 26,911