Changes in Residential and Participation Mortgage Servicing Rights (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Carrying value, beginning of year		$ 228,099
Carrying value, end of period		2,729	$ 228,099
Mortgage Servicing Rights Residential
Finite-Lived Intangible Assets [Line Items]
Carrying value, beginning of year		228,099	243,389
Additions		18,054	45,588
Sales		(208,827)
Due to changes in interest rates		(2,096)	3,341
Due to model assumption changes	[1]		(13,088)
Due to loan payoffs		(22,610)	(33,425)
Due to passage of time and other changes		(9,891)	(17,706)
Carrying value, end of period		2,729	228,099
Mortgage Servicing Rights Participation
Finite-Lived Intangible Assets [Line Items]
Carrying value, beginning of year		5,862	4,345
Additions		710	3,774
Amortization		(3,201)	(2,257)
Carrying value, end of period		$ 3,371	$ 5,862

[1] Represents changes in fair value driven by changes to the inputs to the valuation model related to assumed prepayment speeds.