Rollforward of Cumulative Other Than Temporary Credit Losses Recognized in Earnings for Debt Securities (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Jun. 30, 2012
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Estimated credit losses, beginning of year	$ 3,155	$ 3,247
Credit losses - no previous OTTI recognized	70
Credit losses - previous OTTI recognized	22
Estimated credit losses, end of period	$ 3,247	$ 3,247