Contingent Liabilities and Commitments - Additional Information (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Commitment And Contingencies [Line Items]
Contingent liability	$ 495,395,000	$ 506,888,000	$ 520,375,000	$ 502,611,000
Guarantee of indebtedness, relating to unused letters of credit	5,300,000
Escrow Account Deposit
Commitment And Contingencies [Line Items]
Contingent liability	1,200,000,000
Exchange Traded Funds
Commitment And Contingencies [Line Items]
Contingent liability	$ 1,100,000,000