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, 2019	Mar. 31, 2018
Loans held-for- investment
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 11,347	$ 33,717
Net Income	167	(1,128)
Other comprehensive loss	0	0
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	(465)	(6,255)
Net transfers into/(out of) Level 3	200	286
Ending Balance	11,249	26,620
Mortgage servicing rights
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	75,183	33,676
Net Income	(4,161)	7,090
Other comprehensive loss		0
Purchases		13,806
Issuances	0	0
Settlements		0
Net transfers into/(out of) Level 3	0	0
Ending Balance	71,022	54,572
Derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	2,457	2,157
Net Income	632	2,448
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	3,089	4,605
Municipal Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	108,926	77,181
Net Income		0
Other comprehensive loss	1,537	(2,190)
Purchases	969	12,270
Issuances	0	0
Sales	0	0
Settlements	(7,598)	(2,669)
Net transfers into/(out of) Level 3		0
Ending Balance	103,834	84,592
U.S. Government agencies
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	3,150	3,779
Net Income	0	0
Other comprehensive loss	1	(163)
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	(158)	0
Net transfers into/(out of) Level 3	0	0
Ending Balance	$ 2,993	$ 3,616