Short-Term Borrowings and Long-Term Debt - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Debt Disclosure [Line Items]
Interest expense on debt	$ 5.8	$ 5.7	$ 17.4	$ 17.4
Weighted average interest rates associated with short-term borrowings	0.07%		0.07%
Federal Home Loan Bank Advances | San Francisco
Debt Disclosure [Line Items]
Market value of collateral pledged	1,200		1,200
Federal Reserve Bank Advances
Debt Disclosure [Line Items]
Market value of collateral pledged	$ 517		$ 517