Short-Term Borrowings - Short-Term Borrowings (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Federal Funds Purchased [Member]
Short-term Debt [Line Items]
Amount outstanding at period-end	$ 12,000	$ 7,725
Average amount outstanding during period	12,196	32,960
Maximum amount at any month-end during period	12,000	37,320
Weighted average interest at period-end	0.13%	0.13%
Weighted average interest rate during period	0.25%	0.22%
Securities Sold Under Agreements to Repurchase [Member]
Short-term Debt [Line Items]
Amount outstanding at period-end	624,573	650,235
Average amount outstanding during period	688,704	763,259
Maximum amount at any month-end during period	816,617	797,615
Weighted average interest at period-end	0.03%	0.64%
Weighted average interest rate during period	0.27%	0.58%
FHLB Borrowings [Member]
Short-term Debt [Line Items]
Amount outstanding at period-end	515,000
Average amount outstanding during period	304,781	9,863
Maximum amount at any month-end during period	$ 565,000
Weighted average interest at period-end	0.12%
Weighted average interest rate during period	0.15%	0.18%