Changes in Fair Value of Loans Held for Sale (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Option, Quantitative Disclosures [Line Items]
Mortgage Banking Income	[1]	$ (19,177)	$ (33,069)	$ (6,082)
Loans held for sale
Fair Value, Option, Quantitative Disclosures [Line Items]
Mortgage Banking Income	[1]	899	(5,616)	(472)
Mortgage servicing rights
Fair Value, Option, Quantitative Disclosures [Line Items]
Mortgage Banking Income	[1]	$ (20,076)	$ (27,453)	$ (5,610)

[1] Does not include the effect of hedging activities, which is included in "Other non-interest income."