Remaining Contractual Maturities Of Repurchase Agreements (Detail) $ in Thousands	Jun. 30, 2018 USD ($)
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	$ 1,287,474
2 to 29 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	1,282,974
Over 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	4,500
U.S. Treasury [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	15,755
U.S. Treasury [Member] | 2 to 29 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	15,755
U.S. Agencies [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	1,271,719
U.S. Agencies [Member] | 2 to 29 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	1,267,219
U.S. Agencies [Member] | Over 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	$ 4,500