Summary of Significant Accounting Policies - Additional Information (Detail)		3 Months Ended	12 Months Ended
	Oct. 16, 2009 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2016 USD ($) Segment Office Portfolios
Summary Of Significant Accounting Policies [Line Items]
Number of operating business segments | Segment			2
Fair value of securities		$ 898,600,000
Net unrealized holding gain after tax		$ 3,900,000
Number of days on which collection of interest on principal is no longer probable			90 days
Period over which borrower has to demonstrate repayment performance in compliance with restructured terms to take loan out of nonaccrual status			6 months
Original terms of modified loans			6 months
Number of days on which charge-offs unsecured consumer loans are recorded			120 days
Minimum loan amount for credit review process			$ 1,000,000
Allowance for loan losses look-back period, revised			Rolling 20-quarters
Number of loan segments with similar risk characteristics | Portfolios			8
Agreed percentage of losses in loss sharing agreement	80.00%
Loss amount with respect to covered assets	$ 26,700,000
Agreed percentage of reimbursement in loss sharing agreement	95.00%
Commercial loans expiration date			Oct. 16, 2014
Goodwill impairment			$ 0
Number of offices providing services, CitizensTrust | Office			3
Assets under administration, CitizensTrust			$ 2,690,000,000
Assets under management, CitizensTrust			$ 2,080,000,000
Commercial Loans [Member]
Summary Of Significant Accounting Policies [Line Items]
Loss sharing agreement term	5 years
Loss recovery provisions term	8 years
Single-family Residential Loans [Member]
Summary Of Significant Accounting Policies [Line Items]
Loss sharing agreement term	10 years
Loss recovery provisions term	10 years
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of days over which principal or interest payments are past due to consider loans, excluding PCI loans			30 days
Number of days for which principal or interest payments on loans, excluding PCI loans remain accrual			30 days
Length of modified debt term			3 months
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of days for which principal or interest payments on loans, excluding PCI loans remain accrual			89 days
Length of modified debt term			12 months
Loss recoveries on covered assets	$ 144,000,000