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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Loans held-for- investment
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 13,764	$ 30,173	$ 33,717	$ 22,137
Net Income	(32)	177	(1,420)	1,369
Other comprehensive loss	0	0	0	0
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	(1,520)	(4,504)	(24,177)	(9,995)
Net transfers into/(out of) Level 3	1,244	3,858	5,336	16,193
Ending Balance	13,456	29,704	13,456	29,704
Mortgage servicing rights
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	63,194	27,307	33,676	19,103
Net Income	11,336	2,107	27,048	10,311
Other comprehensive loss		0		0
Purchases		0	13,806	0
Issuances	0	0	0	0
Settlements		0		0
Net transfers into/(out of) Level 3	0	0	0	0
Ending Balance	74,530	29,414	74,530	29,414
Derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	3,819	1,047	2,157	2,291
Net Income	(1,327)	169	335	(1,075)
Other comprehensive loss	0	0	0	0
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Net transfers into/(out of) Level 3	0	0	0	0
Ending Balance	2,492	1,216	2,492	1,216
Municipal Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	96,566	77,341	77,181	79,626
Net Income		0		0
Other comprehensive loss	(2,573)	(4,113)	(5,658)	(1,084)
Purchases	4,830	0	31,846	10,879
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	(1,237)	(4,845)	(5,783)	(21,038)
Net transfers into/(out of) Level 3		0		0
Ending Balance	97,586	68,383	97,586	68,383
U.S. Government agencies
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	3,482	4,110	3,779	0
Net Income	0	0	0	0
Other comprehensive loss	(9)	(167)	(306)	(340)
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	(158)	0	(158)	0
Net transfers into/(out of) Level 3	0	0	0	4,283
Ending Balance	$ 3,315	$ 3,943	$ 3,315	$ 3,943