Securities Available For Sale (Narrative) (Detail) (USD $)	9 Months Ended		27 Months Ended		36 Months Ended		3 Months Ended			9 Months Ended			36 Months Ended								9 Months Ended			9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011 Auction Rate Securities [Member]		Sep. 30, 2012 Auction Rate Securities [Member]		Sep. 30, 2012 Auction Rate Securities [Member] Collateralized Debt Obligations [Member] Y		Sep. 30, 2011 Auction Rate Securities [Member] Collateralized Debt Obligations [Member]	Sep. 30, 2012 Auction Rate Securities [Member] Collateralized Debt Obligations [Member] Y		Sep. 30, 2011 Auction Rate Securities [Member] Collateralized Debt Obligations [Member]	Sep. 30, 2012 Auction Rate Securities [Member] Collateralized Debt Obligations [Member] Y		Dec. 31, 2011 Auction Rate Securities [Member] Collateralized Debt Obligations [Member]		Sep. 30, 2012 Auction Rate Securities [Member] Asset-backed Securities [Member]		Dec. 31, 2011 Auction Rate Securities [Member] Asset-backed Securities [Member]		Sep. 30, 2012 Minimum [Member] Auction Rate Securities [Member]	Sep. 30, 2012 Minimum [Member] Auction Rate Securities [Member] Collateralized Debt Obligations [Member]	Sep. 30, 2012 Minimum [Member] Auction Rate Securities [Member] Asset-backed Securities [Member] Y	Sep. 30, 2012 Maximum [Member] Auction Rate Securities [Member]	Sep. 30, 2012 Maximum [Member] Auction Rate Securities [Member] Collateralized Debt Obligations [Member]	Sep. 30, 2012 Maximum [Member] Auction Rate Securities [Member] Asset-backed Securities [Member] Y
Schedule of Available-for-sale Securities [Line Items]
Investment, debt security, default rate of underlying assets										6.23%												8.00%			9.00%
Auction-rate securities yield rate																					1.50%			2.48%
Cumulative credit and other losses recognized in earnings	$ 0	$ 457,000	$ 3,300,000		$ 3,300,000								$ 3,300,000
Other-than-temporary impairment losses in earnings							0		500,000	0		500,000
Cost basis			14,600,000	[1]	14,600,000	[1]	1,700,000	[1]		1,700,000	[1]		1,700,000	[1]	1,700,000	[1]	12,900,000	[1]	12,900,000	[1]
Par value of securities held			$ 17,900,000		$ 17,900,000		$ 5,000,000			$ 5,000,000			$ 5,000,000
Contractual maturities of securities, in years							4			4			4										15			35

[1] Adjusted cost basis reflects adjustments for credit and other losses recognized in earnings. Cumulative adjustments to the cost basis of securities held as of both September 30, 2012 and December 31, 2011 totaled $3.3 million. Par value of securities held as of both September 30, 2012 and December 31, 2011 was $17.9 million.