Loans - Carrying Amount of Purchased Credit Impaired Loans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable Impaired [Line Items]
Beginning Balance	$ 1,519,477
Loans purchased during the year	53,633	$ 44,311	$ 15,154
Ending Balance	2,016,141	$ 1,519,477
Purchased Credit Impaired Loans [Member]
Financing Receivable Impaired [Line Items]
Loans purchased during the year	1,797
Ending Balance	$ 1,194