Allowance for Credit Losses - Summary of Activity in Allowance for Loan Losses (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Mar. 31, 2014	Mar. 31, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance			$ 113,617	$ 130,200	$ 103,053	$ 117,444
Loans charged-off	(2,345)	(12,006)	(21,335)	(32,081)
Charged-off loans recovered	1,730	2,867	10,878	6,006
Net (charge-offs) recoveries	(615)	(9,139)	(10,457)	(26,075)
Provision for loan losses	0	4,280	(722)	8,460
Ending balance	102,438	112,585	102,438	112,585	103,053	117,444
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance			51,551	64,370	48,421	53,732
Loans charged-off	(1,852)	(8,556)	(11,806)	(17,322)
Charged-off loans recovered	1,190	1,103	6,154	3,043
Net (charge-offs) recoveries	(662)	(7,453)	(5,652)	(14,279)
Provision for loan losses	(1,388)	3,453	472	(359)
Ending balance	46,371	49,732	46,371	49,732	48,421	53,732
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance			42,343	44,069	35,267	43,179
Loans charged-off	(181)	(947)	(6,703)	(7,329)
Charged-off loans recovered	26	896	2,831	961
Net (charge-offs) recoveries	(155)	(51)	(3,872)	(6,368)
Provision for loan losses	1,954	184	(1,405)	5,611
Ending balance	37,066	43,312	37,066	43,312	35,267	43,179
Residential Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance			7,786	9,423	7,018	8,521
Loans charged-off	(240)	(780)	(515)	(3,338)
Charged-off loans recovered	8	230	244	368
Net (charge-offs) recoveries	(232)	(550)	(271)	(2,970)
Provision for loan losses	(610)	56	(1,339)	1,574
Ending balance	6,176	8,027	6,176	8,027	7,018	8,521
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance			4,359	5,542	5,368	5,084
Loans charged-off	(72)	(1,723)	(2,311)	(4,092)
Charged-off loans recovered	506	638	1,649	1,634
Net (charge-offs) recoveries	434	(1,085)	(662)	(2,458)
Provision for loan losses	(22)	80	2,083	995
Ending balance	5,780	4,079	5,780	4,079	5,368	5,084
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance			7,578	6,796	6,979	6,928
Loans charged-off	0	0	0	0
Charged-off loans recovered	0	0	0	0
Net (charge-offs) recoveries	0	0	0	0
Provision for loan losses	66	507	(533)	639
Ending balance	$ 7,045	$ 7,435	$ 7,045	$ 7,435	$ 6,979	$ 6,928