Outstanding Short Term Borrowings and Long Term Debt (Detail) (USD $) In Thousands, unless otherwise specified					12 Months Ended						12 Months Ended			3 Months Ended						12 Months Ended				12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2012 Federal Funds Purchased	Dec. 31, 2011 Federal Funds Purchased	Dec. 31, 2012 Other short-term borrowings		Dec. 31, 2011 Other short-term borrowings		Dec. 31, 2012 5.375% Senior Notes	Dec. 31, 2011 5.375% Senior Notes	Sep. 30, 2010 5.375% Senior Notes	Jun. 30, 2012 5.70% Senior Notes		Dec. 31, 2012 5.70% Senior Notes	May 31, 2012 5.70% Senior Notes	Dec. 31, 2011 5.70% Senior Notes		Dec. 31, 2012 6.05% Subordinated Notes		Dec. 31, 2011 6.05% Subordinated Notes		Dec. 31, 2012 7.0% Junior Subordinated Debentures	Dec. 31, 2011 7.0% Junior Subordinated Debentures
Debt Outstanding [Line Items]
Maturity					Jan. 01, 2013						Sep. 15, 2020			Jun. 01, 2012	[1]					Jun. 01, 2017	[2]			Oct. 15, 2033
Principal value	$ 0	[3]			$ 160,000		$ 6,110	[4]			$ 350,000		$ 350,000			$ 0	$ 141,000			$ 45,964	[2]			$ 50,000
Short-term borrowings	166,110		0		160,000	0	6,110	[4]	0	[4]
Senior Notes											347,995	347,793				0		143,969	[1]
Subordinated Notes																				54,571	[2]	55,075	[2]
Junior Subordinated Debentures																								55,196	55,372
Other long-term debt	0	[3]	1,439	[3]
Total long-term debt	$ 457,762		$ 603,648

[1] At December 31, 2011, included in the carrying value of our 5.70% Senior Notes was $2.6 million related to hedge accounting associated with the notes.

[2] At December 31, 2012 and 2011, included in the carrying value of our 6.05% Subordinated Notes were $9.0 million and $8.8 million, respectively, related to hedge accounting associated with the notes.

[3] Represents long-term notes payable related to one of our debt fund investments. The last payment related to the notes was made in April 2012.

[4] Represents cash collateral received from our counterparty for our interest rate swap agreement related to our 6.05% Subordinated Notes.