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		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Available for Sale Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance, beginning of the period	$ 77,920	$ 84,720	$ 71,674	$ 77,311
Total net gains (losses) for the period included in, Net income		(550)		(550)
Total net gains (losses) for the period included in, Other comprehensive income	3,406	7,847	11,439	18,954
Settlements	(2,214)	(2,926)	(4,001)	(6,624)
Balance, end of the period	79,112	89,091	79,112	89,091
Change in unrealized losses for the period included in earnings for assets held at the end of the reporting period		$ (550)		$ (550)