Business Segments - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014 Segment	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Operating segments	2
Trust and Investment Services [Member]
Segment Reporting Information [Line Items]
Market value of assets held by trust and investment services segment	3,800,000,000	$ 3,700,000,000	$ 3,200,000,000
Total non-fiduciary assets of the trust and investment services segment	4,000,000	$ 3,900,000	$ 2,800,000