Short-Term Borrowings - Short-Term Borrowings (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Federal Funds Purchased [Member]
Short-term Debt [Line Items]
Amount outstanding at period-end	$ 10,100	$ 12,000
Average amount outstanding during period	15,992	12,196
Maximum amount at any month-end during period	$ 13,675	$ 12,000
Weighted-average interest at period-end	0.13%	0.13%
Weighted-average interest rate during period	0.26%	0.25%
Securities Sold Under Agreements to Repurchase [Member]
Short-term Debt [Line Items]
Amount outstanding at period-end	$ 513,544	$ 624,573
Average amount outstanding during period	539,169	688,704
Maximum amount at any month-end during period	$ 609,671	$ 816,617
Weighted-average interest at period-end	0.03%	0.03%
Weighted-average interest rate during period	0.03%	0.27%
FHLB Borrowings [Member]
Short-term Debt [Line Items]
Amount outstanding at period-end	$ 900,000	$ 515,000
Average amount outstanding during period	469,973	304,781
Maximum amount at any month-end during period	$ 900,000	$ 565,000
Weighted-average interest at period-end	0.32%	0.12%
Weighted-average interest rate during period	0.18%	0.15%