Securities - Schedule of Securities Pledged as Collateral (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Financial Instruments Owned and Pledged as Collateral [Abstract]
Securities pledged (carrying value), Collateral for public deposits, trust deposits and for other purposes as required by law	$ 3,145,072	$ 2,779,335
Securities pledged (carrying value), Collateral for short-term borrowings	$ 323,658	$ 322,038
Securities pledged as a percent of total securities	62.40%	67.90%

The carrying amount (fair value) as of the balance sheet date of securities held that had been pledged by counterparties under financing and lending arrangements, pursuant to which the entity has the right by agreement or custom to sell or re-pledge such securities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5086-111524

Name:	us-gaap_SecuritiesHeldAsCollateralAtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant