Financial Instruments with Off-Balance-Sheet Risk - Schedule of Financial Instruments Outstanding Contract Amounts Represent Credit Risk (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Commitments to fund loans	$ 333,874	$ 244,784
Unfunded commitments to fund mortgage warehouse loans	1,567,139	1,230,596
Unfunded commitments under lines of credit	485,345	480,446
Letters of credit	39,890	40,223
Other unused commitments	$ 6,679	$ 5,310

Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions.

No definition available.

Name:	us-gaap_OtherCommitment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant