Loans - Carrying Amount of Purchased Credit Impaired Loans (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Financing Receivable Impaired [Line Items]
Beginning Balance	$ 2,016,141
Loans purchased during the period	18,246	$ 27,039
Ending Balance	2,114,932
Purchased Credit Impaired Loans [Member]
Financing Receivable Impaired [Line Items]
Beginning Balance	1,194
Loans purchased during the period		1,797
Ending Balance	$ 1,091	$ 1,438