Fair Values of Assets and Liabilities (Summary of Changes in Level 3 Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Loans held-for- investment
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 33,717	$ 22,137
Net Income	(1,128)	(112)
Other comprehensive loss	0	0
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	(6,255)	(3,332)
Net transfers into/(out of) Level 3	286	9,855
Ending Balance	26,620	28,548
Mortgage servicing rights
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	33,676	19,103
Net Income	7,090	2,493
Other comprehensive loss		0
Purchases	13,806	0
Issuances	0	0
Settlements		0
Net transfers into/(out of) Level 3	0	0
Ending Balance	54,572	21,596
Derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	2,157	2,291
Net Income	2,448	1,291
Other comprehensive loss	0	0
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	0	0
Net transfers into/(out of) Level 3	0	0
Ending Balance	4,605	3,582
Municipal Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	77,181	79,626
Net Income		0
Other comprehensive loss	(2,190)	457
Purchases	12,270	7,586
Issuances	0	0
Sales	0	0
Settlements	(2,669)	(7,924)
Net transfers into/(out of) Level 3		0
Ending Balance	84,592	79,745
U.S. Government agencies
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	3,779	0
Net Income	0	0
Other comprehensive loss	(163)	0
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	0	0
Net transfers into/(out of) Level 3	0	4,283
Ending Balance	$ 3,616	$ 4,283