Remaining Contractual Maturities Of Repurchase Agreements (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	$ 1,437,094	$ 1,800,000
U.S. Treasury
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	7,136
U.S. Agencies
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	1,429,958
On Demand [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	1
On Demand [Member] | U.S. Agencies
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	1
2 to 29 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	1,436,493
2 to 29 days [Member] | U.S. Treasury
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	7,136
2 to 29 days [Member] | U.S. Agencies
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	1,429,357
Over 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	600
Over 90 Days [Member] | U.S. Agencies
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	$ 600