Intangible Assets - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 2,436,131,000	$ 2,436,131,000
Amortization of core deposit intangibles	208,000	2,391,000	$ 5,344,000
Mortgage servicing rights	6,100,000	233,961,000
Mortgage Banking Income
Finite-Lived Intangible Assets [Line Items]
Contractually specified servicing fees, late fees, and ancillary fees earned in exchange for servicing financial assets	1,200,000	1,300,000	$ 941,000
Mortgage Receivable
Finite-Lived Intangible Assets [Line Items]
Impairment of intangible assets	0	0
Unpaid principal balance of loans serviced for others	3,700,000,000	$ 25,100,000,000
Core deposit intangibles
Finite-Lived Intangible Assets [Line Items]
Amortization of core deposit intangibles	$ 208,000
Core deposit intangibles | Maximum
Finite-Lived Intangible Assets [Line Items]
Amortization period for core deposit intangibles (in years)	10 years