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	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Available for sale securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of the period	$ 28,523		$ 71,674		$ 77,311
Total net (losses) gains for the period included in, Net income	0		0		(5,247)
Total net gains (losses) for the period included in, Other comprehensive income	1,648		657		18,482
Sales	(7,718)		(36,681)		(9,146)
Settlements	(3,144)		(7,127)		(9,726)
Balance, end of the period	19,309		28,523		71,674
Change in unrealized losses for the period included in earnings for assets held at the end of the reporting period	$ 0	[1]	$ 0	[1]	$ (5,247)	[1]

[1] Represents the net impairment losses on securities recognized in earnings for the period.