Short-Term Borrowings and Long-Term Debt - Additional Information (Detail) (USD $)	3 Months Ended									12 Months Ended									1 Months Ended	12 Months Ended		0 Months Ended	12 Months Ended			9 Months Ended	12 Months Ended	1 Months Ended	3 Months Ended						1 Months Ended		3 Months Ended			12 Months Ended			12 Months Ended			12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2012 Special Purpose Trust Entity	Dec. 31, 2011 Special Purpose Trust Entity	Dec. 31, 2010 Special Purpose Trust Entity	Sep. 30, 2010 5.375% Senior Notes	Dec. 31, 2012 5.375% Senior Notes	Dec. 31, 2011 5.375% Senior Notes	Apr. 15, 2011 3.875% Convertible Notes	Dec. 31, 2012 3.875% Convertible Notes	Dec. 31, 2011 3.875% Convertible Notes	Mar. 31, 2011 3.875% Convertible Notes	Sep. 30, 2011 3.875% Convertible Notes Hedge and warrant agreement	Dec. 31, 2012 3.875% Convertible Notes Hedge and warrant agreement	May 02, 2011 5.70% 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	May 15, 2007 5.70% Senior Notes	May 02, 2011 6.05% Subordinated Notes	May 15, 2007 6.05% Subordinated Notes	Jun. 30, 2011 Interest rate swaps	Dec. 31, 2012 7.0% Junior Subordinated Debentures	Oct. 30, 2003 7.0% Junior Subordinated Debentures	Dec. 31, 2012 Federal Funds Purchased	Dec. 31, 2012 Federal Home Loan Bank Advances San Francisco	Dec. 31, 2012 Federal Reserve Bank Advances	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 Disclosure [Line Items]
Stated interest rate																				5.375%	5.375%		3.875%	3.875%							5.70%		5.70%										6.05%		6.05%	7.00%	7.00%
Interest expense, debt										$ 24,194,000		$ 30,249,000		$ 28,818,000										$ 4,200,000
Weighted average interest rates associated with short-term borrowings	0.07%									0.07%
Principal value	0	[1]								0	[1]								350,000,000	350,000,000			250,000,000		250,000,000						0	141,000,000						52,000,000		160,000,000			45,964,000	[2]		50,000,000
Discount on issuance of debt																																		250,000,000		250,000,000
Maturity																				Sep. 15, 2020		Apr. 15, 2011	Apr. 15, 2011						Jun. 01, 2012	[3]										Jan. 01, 2013			Jun. 01, 2017	[2]		Oct. 15, 2033
Aggregate conversion settlement payment																						260,000,000	260,000,000
Payments for settlement of 3.875% Convertible Notes										0		250,000,000		0						250,000,000		250,000,000	250,000,000
Debt issuance costs																																							2,200,000
Value of common stock shares issued																						10,000,000	10,000,000
Proceeds from issuance of senior notes, net of discount and issuance cost										0		0		344,476,000					345,000,000
Number of common stock shares issued																						187,760	187,760
Number of shares called																						186,736	186,736
Value of stock called																						10,000,000					10,000,000
Notes repurchase amount																												109,000,000							204,000,000
Gross loss from extinguishment of debt																																					33,900,000
Gross gain on derivative termination																																					37,000,000
Net gain from note repurchases and termination of corresponding interest rate swaps										0	[4]	3,123,000	[4]	0	[4]																						3,100,000
Price per share of the conversion feature for the hedge and warrant agreement associated with our convertible notes																										$ 64.43	$ 64.43
Effective interest rate																								5.92%
Period of warrants ratable expiration beginning July 15, 2011																										60 days	60 days
Distributions	7,605,000		7,528,000	7,884,000	7,837,000	7,732,000	7,869,000	9,708,000	13,802,000	30,854,000		39,111,000		43,596,000		3,500,000	3,500,000	3,500,000
Federal Funds Purchased																																								160,000,000
Market value of collateral pledged																																									1,500,000,000	561,000,000
Unused and available funds																																										560,756,000
Unused and available funds																																									$ 1,482,297,000

[1] 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.

[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] 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.

[4] Represents gains from the repurchase of 109 million of our 5.70% Senior Notes and $204 million of our 6.05% Subordinated Notes and the termination of the corresponding portions of interest rate swaps in 2011. For more information, see Note 11–"Short-Term Borrowings and Long-Term Debt."