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, 2021	Mar. 31, 2020
Loans held-for- investment
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 10,280	$ 9,620
Net Income	(530)	(78)
Other comprehensive income (loss)	0	0
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	(3,918)	(96)
Net transfers into/(out of) Level 3	576	122
Ending Balance	6,408	9,568
Mortgage servicing rights
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	92,081	85,638
Net Income	32,235	(12,134)
Other comprehensive income (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	124,316	73,504
Derivative assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	48,091	2,631
Net Income	(22,457)	37,185
Other comprehensive income (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	25,634	39,816
Municipal Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	109,876	111,950
Net Income	0	0
Other comprehensive income (loss)	(2,068)	(1,249)
Purchases	0	5,875
Issuances	0	0
Sales	0	0
Settlements	(6,060)	(3,309)
Net transfers into/(out of) Level 3	0	0
Ending Balance	101,748	113,267
U.S. Government agencies
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	1,966	2,646
Net Income	0	0
Other comprehensive income (loss)	(24)	(32)
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	(158)	(157)
Net transfers into/(out of) Level 3	0	0
Ending Balance	$ 1,784	$ 2,457